diff --git a/tests/phpunit/tests/compat.php b/tests/phpunit/tests/compat.php
deleted file mode 100644
index ab75d6256b..0000000000
--- a/tests/phpunit/tests/compat.php
+++ /dev/null
@@ -1,325 +0,0 @@
-<?php
-
-/**
- * @group compat
- * @group security-153
- */
-class Tests_Compat extends WP_UnitTestCase {
-	function utf8_string_lengths() {
-		return array(
-			// String, character_length, byte_length.
-			array( 'баба', 4, 8 ),
-			array( 'баб', 3, 6 ),
-			array( 'I am your б', 11, 12 ),
-			array( '1111111111', 10, 10 ),
-			array( '²²²²²²²²²²', 10, 20 ),
-			array( '３３３３３３３３３３', 10, 30 ),
-			array( '𝟜𝟜𝟜𝟜𝟜𝟜𝟜𝟜𝟜𝟜', 10, 40 ),
-			array( '1²３𝟜1²３𝟜1²３𝟜', 12, 30 ),
-		);
-	}
-
-	function utf8_substrings() {
-		return array(
-			// String, start, length, character_substring, byte_substring.
-			array( 'баба', 0, 3, 'баб', "б\xD0" ),
-			array( 'баба', 0, -1, 'баб', "баб\xD0" ),
-			array( 'баба', 1, null, 'аба', "\xB1аба" ),
-			array( 'баба', -3, null, 'аба', "\xB1а" ),
-			array( 'баба', -3, 2, 'аб', "\xB1\xD0" ),
-			array( 'баба', -1, 2, 'а', "\xB0" ),
-			array( 'I am your баба', 0, 11, 'I am your б', "I am your \xD0" ),
-		);
-	}
-
-	/**
-	 * @dataProvider utf8_string_lengths
-	 */
-	function test_mb_strlen( $string, $expected_character_length ) {
-		$this->assertSame( $expected_character_length, _mb_strlen( $string, 'UTF-8' ) );
-	}
-
-	/**
-	 * @dataProvider utf8_string_lengths
-	 */
-	function test_mb_strlen_via_regex( $string, $expected_character_length ) {
-		_wp_can_use_pcre_u( false );
-		$this->assertSame( $expected_character_length, _mb_strlen( $string, 'UTF-8' ) );
-		_wp_can_use_pcre_u( 'reset' );
-	}
-
-	/**
-	 * @dataProvider utf8_string_lengths
-	 */
-	function test_8bit_mb_strlen( $string, $expected_character_length, $expected_byte_length ) {
-		$this->assertSame( $expected_byte_length, _mb_strlen( $string, '8bit' ) );
-	}
-
-	/**
-	 * @dataProvider utf8_substrings
-	 */
-	function test_mb_substr( $string, $start, $length, $expected_character_substring ) {
-		$this->assertSame( $expected_character_substring, _mb_substr( $string, $start, $length, 'UTF-8' ) );
-	}
-
-	/**
-	 * @dataProvider utf8_substrings
-	 */
-	function test_mb_substr_via_regex( $string, $start, $length, $expected_character_substring ) {
-		_wp_can_use_pcre_u( false );
-		$this->assertSame( $expected_character_substring, _mb_substr( $string, $start, $length, 'UTF-8' ) );
-		_wp_can_use_pcre_u( 'reset' );
-	}
-
-	/**
-	 * @dataProvider utf8_substrings
-	 */
-	function test_8bit_mb_substr( $string, $start, $length, $expected_character_substring, $expected_byte_substring ) {
-		$this->assertSame( $expected_byte_substring, _mb_substr( $string, $start, $length, '8bit' ) );
-	}
-
-	function test_mb_substr_phpcore() {
-		/* https://github.com/php/php-src/blob/php-5.6.8/ext/mbstring/tests/mb_substr_basic.phpt */
-		$string_ascii = 'ABCDEF';
-		$string_mb    = base64_decode( '5pel5pys6Kqe44OG44Kt44K544OI44Gn44GZ44CCMDEyMzTvvJXvvJbvvJfvvJjvvJnjgII=' );
-
-		$this->assertSame( 'DEF', _mb_substr( $string_ascii, 3 ) );
-		$this->assertSame( 'DEF', _mb_substr( $string_ascii, 3, 5, 'ISO-8859-1' ) );
-
-		// Specific latin-1 as that is the default the core PHP test operates under.
-		$this->assertSame( 'peacrOiqng==', base64_encode( _mb_substr( $string_mb, 2, 7, 'latin-1' ) ) );
-		$this->assertSame( '6Kqe44OG44Kt44K544OI44Gn44GZ', base64_encode( _mb_substr( $string_mb, 2, 7, 'utf-8' ) ) );
-
-		/* https://github.com/php/php-src/blob/php-5.6.8/ext/mbstring/tests/mb_substr_variation1.phpt */
-		$start     = 0;
-		$length    = 5;
-		$unset_var = 10;
-		unset( $unset_var );
-		$heredoc  = <<<EOT
-hello world
-EOT;
-		$inputs   = array(
-			0,
-			1,
-			12345,
-			-2345,
-			// Float data.
-			10.5,
-			-10.5,
-			12.3456789000e10,
-			12.3456789000E-10,
-			.5,
-			// Null data.
-			null,
-			null,
-			// Boolean data.
-			true,
-			false,
-			true,
-			false,
-			// Empty data.
-			'',
-			'',
-			// String data.
-			'string',
-			'string',
-			$heredoc,
-			// Object data.
-			new ClassA(),
-			// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged -- intentionally undefined data
-			@$undefined_var,
-			// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged -- intentionally unset data
-			@$unset_var,
-		);
-		$outputs  = array(
-			'0',
-			'1',
-			'12345',
-			'-2345',
-			'10.5',
-			'-10.5',
-			'12345',
-			'1.234',
-			'0.5',
-			'',
-			'',
-			'1',
-			'',
-			'1',
-			'',
-			'',
-			'',
-			'strin',
-			'strin',
-			'hello',
-			'Class',
-			'',
-			'',
-		);
-		$iterator = 0;
-		foreach ( $inputs as $input ) {
-			$this->assertSame( $outputs[ $iterator ], _mb_substr( $input, $start, $length ) );
-			$iterator++;
-		}
-
-	}
-
-	function test_hash_hmac_simple() {
-		$this->assertSame( '140d1cb79fa12e2a31f32d35ad0a2723', _hash_hmac( 'md5', 'simple', 'key' ) );
-		$this->assertSame( '993003b95758e0ac2eba451a4c5877eb1bb7b92a', _hash_hmac( 'sha1', 'simple', 'key' ) );
-	}
-
-	function test_hash_hmac_padding() {
-		$this->assertSame( '3c1399103807cf12ec38228614416a8c', _hash_hmac( 'md5', 'simple', '65 character key 65 character key 65 character key 65 character k' ) );
-		$this->assertSame( '4428826d20003e309d6c2a6515891370daf184ea', _hash_hmac( 'sha1', 'simple', '65 character key 65 character key 65 character key 65 character k' ) );
-	}
-
-	function test_hash_hmac_output() {
-		$this->assertSame( array( 1 => '140d1cb79fa12e2a31f32d35ad0a2723' ), unpack( 'H32', _hash_hmac( 'md5', 'simple', 'key', true ) ) );
-		$this->assertSame( array( 1 => '993003b95758e0ac2eba451a4c5877eb1bb7b92a' ), unpack( 'H40', _hash_hmac( 'sha1', 'simple', 'key', true ) ) );
-	}
-
-	function test_json_encode_decode() {
-		$this->expectDeprecation();
-
-		require_once ABSPATH . WPINC . '/class-json.php';
-		$json = new Services_JSON();
-		// Super basic test to verify Services_JSON is intact and working.
-		$this->assertSame( '["foo"]', $json->encodeUnsafe( array( 'foo' ) ) );
-		$this->assertSame( array( 'foo' ), $json->decode( '["foo"]' ) );
-	}
-
-	/**
-	 * Test that is_countable() is always available (either from PHP or WP).
-	 *
-	 * @ticket 43583
-	 */
-	function test_is_countable_availability() {
-		$this->assertTrue( function_exists( 'is_countable' ) );
-	}
-
-	/**
-	 * Test is_countable() polyfill.
-	 *
-	 * @ticket 43583
-	 *
-	 * @dataProvider countable_variable_test_data
-	 *
-	 * @param mixed $variable     Variable to check.
-	 * @param bool  $is_countable The expected return value of PHP 7.3 is_countable() function.
-	 */
-	function test_is_countable_functionality( $variable, $is_countable ) {
-		$this->assertSame( is_countable( $variable ), $is_countable );
-	}
-
-	/**
-	 * Data provider for test_is_countable_functionality().
-	 *
-	 * @ticket 43583
-	 *
-	 * @return array {
-	 *     @type array {
-	 *         @type mixed $variable     Variable to check.
-	 *         @type bool  $is_countable The expected return value of PHP 7.3 is_countable() function.
-	 *     }
-	 * }
-	 */
-	public function countable_variable_test_data() {
-		return array(
-			array( true, false ),
-			array( new stdClass(), false ),
-			array( new ArrayIteratorFake(), true ),
-			array( new CountableFake(), true ),
-			array( 16, false ),
-			array( null, false ),
-			array( array( 1, 2, 3 ), true ),
-			array( (array) 1, true ),
-			array( (object) array( 'foo', 'bar', 'baz' ), false ),
-		);
-	}
-
-	/**
-	 * Test is_countable() polyfill for ResourceBundle.
-	 *
-	 * @ticket 43583
-	 *
-	 * @requires extension intl
-	 */
-	function test_is_countable_ResourceBundle() {
-		$this->assertTrue( is_countable( new ResourceBundle( 'en', null ) ) );
-	}
-
-	/**
-	 * Test is_countable() polyfill for SimpleXMLElement.
-	 *
-	 * @ticket 43583
-	 *
-	 * @requires extension simplexml
-	 */
-	function test_is_countable_SimpleXMLElement() {
-		$this->assertTrue( is_countable( new SimpleXMLElement( '<xml><tag>1</tag><tag>2</tag></xml>' ) ) );
-	}
-
-	/**
-	 * Test that is_iterable() is always available (either from PHP or WP).
-	 *
-	 * @ticket 43619
-	 */
-	function test_is_iterable_availability() {
-		$this->assertTrue( function_exists( 'is_iterable' ) );
-	}
-
-	/**
-	 * Test is_iterable() polyfill.
-	 *
-	 * @ticket 43619
-	 *
-	 * @dataProvider iterable_variable_test_data
-	 *
-	 * @param mixed $variable    Variable to check.
-	 * @param bool  $is_iterable The expected return value of PHP 7.1 is_iterable() function.
-	 */
-	function test_is_iterable_functionality( $variable, $is_iterable ) {
-		$this->assertSame( is_iterable( $variable ), $is_iterable );
-	}
-
-	/**
-	 * Data provider for test_is_iterable_functionality().
-	 *
-	 * @ticket 43619
-	 *
-	 * @return array {
-	 *     @type array {
-	 *         @type mixed $variable    Variable to check.
-	 *         @type bool  $is_iterable The expected return value of PHP 7.1 is_iterable() function.
-	 *     }
-	 * }
-	 */
-	public function iterable_variable_test_data() {
-		return array(
-			array( array(), true ),
-			array( array( 1, 2, 3 ), true ),
-			array( new ArrayIterator( array( 1, 2, 3 ) ), true ),
-			array( 1, false ),
-			array( 3.14, false ),
-			array( new stdClass(), false ),
-		);
-	}
-}
-
-/* used in test_mb_substr_phpcore */
-class ClassA {
-	public function __toString() {
-		return 'Class A object';
-	}
-}
-
-class ArrayIteratorFake extends ArrayIterator {
-}
-
-class CountableFake implements Countable {
-	#[ReturnTypeWillChange]
-	public function count() {
-		return 16;
-	}
-}
diff --git a/tests/phpunit/tests/compat/hashHmac.php b/tests/phpunit/tests/compat/hashHmac.php
new file mode 100644
index 0000000000..f9a61d5fac
--- /dev/null
+++ b/tests/phpunit/tests/compat/hashHmac.php
@@ -0,0 +1,65 @@
+<?php
+
+/**
+ * @group compat
+ *
+ * @covers ::hash_hmac
+ * @covers ::_hash_hmac
+ */
+class Tests_Compat_hashHmac extends WP_UnitTestCase {
+
+	/**
+	 * Test that hash_hmac() is always available (either from PHP or WP).
+	 */
+	public function test_hash_hmac_availability() {
+		$this->assertTrue( function_exists( 'hash_hmac' ) );
+	}
+
+	public function test_hash_hmac_simple() {
+		$data = 'simple';
+		$key  = 'key';
+
+		$this->assertSame(
+			'140d1cb79fa12e2a31f32d35ad0a2723',
+			_hash_hmac( 'md5', $data, $key ),
+			'MD5 hash does not match'
+		);
+		$this->assertSame(
+			'993003b95758e0ac2eba451a4c5877eb1bb7b92a',
+			_hash_hmac( 'sha1', $data, $key ),
+			'sha1 hash does not match'
+		);
+	}
+
+	public function test_hash_hmac_padding() {
+		$data = 'simple';
+		$key  = '65 character key 65 character key 65 character key 65 character k';
+
+		$this->assertSame(
+			'3c1399103807cf12ec38228614416a8c',
+			_hash_hmac( 'md5', $data, $key ),
+			'MD5 hash does not match'
+		);
+		$this->assertSame(
+			'4428826d20003e309d6c2a6515891370daf184ea',
+			_hash_hmac( 'sha1', $data, $key ),
+			'sha1 hash does not match'
+		);
+	}
+
+	public function test_hash_hmac_output() {
+		$data = 'simple';
+		$key  = 'key';
+
+		$this->assertSame(
+			array( 1 => '140d1cb79fa12e2a31f32d35ad0a2723' ),
+			unpack( 'H32', _hash_hmac( 'md5', $data, $key, true ) ),
+			'unpacked MD5 hash does not match'
+		);
+		$this->assertSame(
+			array( 1 => '993003b95758e0ac2eba451a4c5877eb1bb7b92a' ),
+			unpack( 'H40', _hash_hmac( 'sha1', $data, $key, true ) ),
+			'unpacked sha1 hash does not match'
+		);
+	}
+}
diff --git a/tests/phpunit/tests/compat/isCountable.php b/tests/phpunit/tests/compat/isCountable.php
new file mode 100644
index 0000000000..66bf3c8c6c
--- /dev/null
+++ b/tests/phpunit/tests/compat/isCountable.php
@@ -0,0 +1,117 @@
+<?php
+
+/**
+ * @group compat
+ *
+ * @covers ::is_countable
+ */
+class Tests_Compat_isCountable extends WP_UnitTestCase {
+
+	/**
+	 * Test that is_countable() is always available (either from PHP or WP).
+	 *
+	 * @ticket 43583
+	 */
+	public function test_is_countable_availability() {
+		$this->assertTrue( function_exists( 'is_countable' ) );
+	}
+
+	/**
+	 * Test is_countable() polyfill.
+	 *
+	 * @ticket 43583
+	 *
+	 * @dataProvider data_is_countable_functionality
+	 *
+	 * @param mixed $variable     Variable to check.
+	 * @param bool  $is_countable The expected return value of PHP 7.3 is_countable() function.
+	 */
+	public function test_is_countable_functionality( $variable, $is_countable ) {
+		$this->assertSame( $is_countable, is_countable( $variable ) );
+	}
+
+	/**
+	 * Data provider for test_is_countable_functionality().
+	 *
+	 * @ticket 43583
+	 *
+	 * @return array {
+	 *     @type array {
+	 *         @type mixed $variable     Variable to check.
+	 *         @type bool  $is_countable The expected return value of PHP 7.3 is_countable() function.
+	 *     }
+	 * }
+	 */
+	public function data_is_countable_functionality() {
+		return array(
+			'boolean true'                     => array(
+				'variable'     => true,
+				'is_countable' => false,
+			),
+			'plain stdClass object'            => array(
+				'variable'     => new stdClass(),
+				'is_countable' => false,
+			),
+			'Array iterator object'            => array(
+				'variable'     => new ArrayIteratorFakeForIsCountable(),
+				'is_countable' => true,
+			),
+			'Countable object'                 => array(
+				'variable'     => new CountableFakeForIsCountable(),
+				'is_countable' => true,
+			),
+			'integer 16'                       => array(
+				'variable'     => 16,
+				'is_countable' => false,
+			),
+			'null'                             => array(
+				'variable'     => null,
+				'is_countable' => false,
+			),
+			'non-empty array, 3 items'         => array(
+				'variable'     => array( 1, 2, 3 ),
+				'is_countable' => true,
+			),
+			'non-empty array, 1 item via cast' => array(
+				'variable'     => (array) 1,
+				'is_countable' => true,
+			),
+			'array cast to object'             => array(
+				'variable'     => (object) array( 'foo', 'bar', 'baz' ),
+				'is_countable' => false,
+			),
+		);
+	}
+
+	/**
+	 * Test is_countable() polyfill for ResourceBundle.
+	 *
+	 * @ticket 43583
+	 *
+	 * @requires extension intl
+	 */
+	public function test_is_countable_ResourceBundle() {
+		$this->assertTrue( is_countable( new ResourceBundle( 'en', null ) ) );
+	}
+
+	/**
+	 * Test is_countable() polyfill for SimpleXMLElement.
+	 *
+	 * @ticket 43583
+	 *
+	 * @requires extension simplexml
+	 */
+	public function test_is_countable_SimpleXMLElement() {
+		$this->assertTrue( is_countable( new SimpleXMLElement( '<xml><tag>1</tag><tag>2</tag></xml>' ) ) );
+	}
+}
+
+class ArrayIteratorFakeForIsCountable extends ArrayIterator {
+}
+
+class CountableFakeForIsCountable implements Countable {
+	#[ReturnTypeWillChange]
+	public function count() {
+		return 16;
+	}
+}
diff --git a/tests/phpunit/tests/compat/isIterable.php b/tests/phpunit/tests/compat/isIterable.php
new file mode 100644
index 0000000000..e09bb18278
--- /dev/null
+++ b/tests/phpunit/tests/compat/isIterable.php
@@ -0,0 +1,77 @@
+<?php
+
+/**
+ * @group compat
+ *
+ * @covers ::is_iterable
+ */
+class Tests_Compat_isIterable extends WP_UnitTestCase {
+
+	/**
+	 * Test that is_iterable() is always available (either from PHP or WP).
+	 *
+	 * @ticket 43619
+	 */
+	public function test_is_iterable_availability() {
+		$this->assertTrue( function_exists( 'is_iterable' ) );
+	}
+
+	/**
+	 * Test is_iterable() polyfill.
+	 *
+	 * @ticket 43619
+	 *
+	 * @dataProvider data_is_iterable_functionality
+	 *
+	 * @param mixed $variable    Variable to check.
+	 * @param bool  $is_iterable The expected return value of PHP 7.1 is_iterable() function.
+	 */
+	public function test_is_iterable_functionality( $variable, $is_iterable ) {
+		$this->assertSame( $is_iterable, is_iterable( $variable ) );
+	}
+
+	/**
+	 * Data provider for test_is_iterable_functionality().
+	 *
+	 * @ticket 43619
+	 *
+	 * @return array {
+	 *     @type array {
+	 *         @type mixed $variable    Variable to check.
+	 *         @type bool  $is_iterable The expected return value of PHP 7.1 is_iterable() function.
+	 *     }
+	 * }
+	 */
+	public function data_is_iterable_functionality() {
+		return array(
+			'empty array'           => array(
+				'variable'    => array(),
+				'is_iterable' => true,
+			),
+			'non-empty array'       => array(
+				'variable'    => array( 1, 2, 3 ),
+				'is_iterable' => true,
+			),
+			'Iterator object'       => array(
+				'variable'    => new ArrayIterator( array( 1, 2, 3 ) ),
+				'is_iterable' => true,
+			),
+			'null'                  => array(
+				'variable'    => null,
+				'is_iterable' => false,
+			),
+			'integer 1'             => array(
+				'variable'    => 1,
+				'is_iterable' => false,
+			),
+			'float 3.14'            => array(
+				'variable'    => 3.14,
+				'is_iterable' => false,
+			),
+			'plain stdClass object' => array(
+				'variable'    => new stdClass(),
+				'is_iterable' => false,
+			),
+		);
+	}
+}
diff --git a/tests/phpunit/tests/compat/jsonEncodeDecode.php b/tests/phpunit/tests/compat/jsonEncodeDecode.php
new file mode 100644
index 0000000000..475af85b9f
--- /dev/null
+++ b/tests/phpunit/tests/compat/jsonEncodeDecode.php
@@ -0,0 +1,28 @@
+<?php
+
+/**
+ * @group compat
+ *
+ * @covers Services_JSON
+ */
+class Tests_Compat_jsonEncodeDecode extends WP_UnitTestCase {
+
+	public function test_json_encode_decode() {
+		$this->expectDeprecation();
+
+		require_once ABSPATH . WPINC . '/class-json.php';
+		$json = new Services_JSON();
+
+		// Super basic test to verify Services_JSON is intact and working.
+		$this->assertSame(
+			'["foo"]',
+			$json->encodeUnsafe( array( 'foo' ) ),
+			'encodeUnsafe() did not return expected output'
+		);
+		$this->assertSame(
+			array( 'foo' ),
+			$json->decode( '["foo"]' ),
+			'decode() did not return expected output'
+		);
+	}
+}
diff --git a/tests/phpunit/tests/compat/mbStrlen.php b/tests/phpunit/tests/compat/mbStrlen.php
new file mode 100644
index 0000000000..e34b44534b
--- /dev/null
+++ b/tests/phpunit/tests/compat/mbStrlen.php
@@ -0,0 +1,91 @@
+<?php
+
+/**
+ * @group compat
+ * @group security-153
+ *
+ * @covers ::mb_strlen
+ * @covers ::_mb_strlen
+ */
+class Tests_Compat_mbStrlen extends WP_UnitTestCase {
+
+	/**
+	 * Test that mb_strlen() is always available (either from PHP or WP).
+	 */
+	public function test_mb_strlen_availability() {
+		$this->assertTrue( function_exists( 'mb_strlen' ) );
+	}
+
+	/**
+	 * @dataProvider utf8_string_lengths
+	 */
+	public function test_mb_strlen( $string, $expected_character_length ) {
+		$this->assertSame( $expected_character_length, _mb_strlen( $string, 'UTF-8' ) );
+	}
+
+	/**
+	 * @dataProvider utf8_string_lengths
+	 */
+	public function test_mb_strlen_via_regex( $string, $expected_character_length ) {
+		_wp_can_use_pcre_u( false );
+		$this->assertSame( $expected_character_length, _mb_strlen( $string, 'UTF-8' ) );
+		_wp_can_use_pcre_u( 'reset' );
+	}
+
+	/**
+	 * @dataProvider utf8_string_lengths
+	 */
+	public function test_8bit_mb_strlen( $string, $expected_character_length, $expected_byte_length ) {
+		$this->assertSame( $expected_byte_length, _mb_strlen( $string, '8bit' ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function utf8_string_lengths() {
+		return array(
+			array(
+				'string'                    => 'баба',
+				'expected_character_length' => 4,
+				'expected_byte_length'      => 8,
+			),
+			array(
+				'string'                    => 'баб',
+				'expected_character_length' => 3,
+				'expected_byte_length'      => 6,
+			),
+			array(
+				'string'                    => 'I am your б',
+				'expected_character_length' => 11,
+				'expected_byte_length'      => 12,
+			),
+			array(
+				'string'                    => '1111111111',
+				'expected_character_length' => 10,
+				'expected_byte_length'      => 10,
+			),
+			array(
+				'string'                    => '²²²²²²²²²²',
+				'expected_character_length' => 10,
+				'expected_byte_length'      => 20,
+			),
+			array(
+				'string'                    => '３３３３３３３３３３',
+				'expected_character_length' => 10,
+				'expected_byte_length'      => 30,
+			),
+			array(
+				'string'                    => '𝟜𝟜𝟜𝟜𝟜𝟜𝟜𝟜𝟜𝟜',
+				'expected_character_length' => 10,
+				'expected_byte_length'      => 40,
+			),
+			array(
+				'string'                    => '1²３𝟜1²３𝟜1²３𝟜',
+				'expected_character_length' => 12,
+				'expected_byte_length'      => 30,
+			),
+		);
+	}
+}
diff --git a/tests/phpunit/tests/compat/mbSubstr.php b/tests/phpunit/tests/compat/mbSubstr.php
new file mode 100644
index 0000000000..f03f5f0f19
--- /dev/null
+++ b/tests/phpunit/tests/compat/mbSubstr.php
@@ -0,0 +1,241 @@
+<?php
+
+/**
+ * @group compat
+ * @group security-153
+ *
+ * @covers ::mb_substr
+ * @covers ::_mb_substr
+ */
+class Tests_Compat_mbSubstr extends WP_UnitTestCase {
+
+	/**
+	 * Test that mb_substr() is always available (either from PHP or WP).
+	 */
+	public function test_mb_substr_availability() {
+		$this->assertTrue( function_exists( 'mb_substr' ) );
+	}
+
+	/**
+	 * @dataProvider utf8_substrings
+	 */
+	public function test_mb_substr( $string, $start, $length, $expected_character_substring ) {
+		$this->assertSame( $expected_character_substring, _mb_substr( $string, $start, $length, 'UTF-8' ) );
+	}
+
+	/**
+	 * @dataProvider utf8_substrings
+	 */
+	public function test_mb_substr_via_regex( $string, $start, $length, $expected_character_substring ) {
+		_wp_can_use_pcre_u( false );
+		$this->assertSame( $expected_character_substring, _mb_substr( $string, $start, $length, 'UTF-8' ) );
+		_wp_can_use_pcre_u( 'reset' );
+	}
+
+	/**
+	 * @dataProvider utf8_substrings
+	 */
+	public function test_8bit_mb_substr( $string, $start, $length, $expected_character_substring, $expected_byte_substring ) {
+		$this->assertSame( $expected_byte_substring, _mb_substr( $string, $start, $length, '8bit' ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function utf8_substrings() {
+		return array(
+			array(
+				'string'                       => 'баба',
+				'start'                        => 0,
+				'length'                       => 3,
+				'expected_character_substring' => 'баб',
+				'expected_byte_substring'      => "б\xD0",
+			),
+			array(
+				'string'                       => 'баба',
+				'start'                        => 0,
+				'length'                       => -1,
+				'expected_character_substring' => 'баб',
+				'expected_byte_substring'      => "баб\xD0",
+			),
+			array(
+				'string'                       => 'баба',
+				'start'                        => 1,
+				'length'                       => null,
+				'expected_character_substring' => 'аба',
+				'expected_byte_substring'      => "\xB1аба",
+			),
+			array(
+				'string'                       => 'баба',
+				'start'                        => -3,
+				'length'                       => null,
+				'expected_character_substring' => 'аба',
+				'expected_byte_substring'      => "\xB1а",
+			),
+			array(
+				'string'                       => 'баба',
+				'start'                        => -3,
+				'length'                       => 2,
+				'expected_character_substring' => 'аб',
+				'expected_byte_substring'      => "\xB1\xD0",
+			),
+			array(
+				'string'                       => 'баба',
+				'start'                        => -1,
+				'length'                       => 2,
+				'expected_character_substring' => 'а',
+				'expected_byte_substring'      => "\xB0",
+			),
+			array(
+				'string'                       => 'I am your баба',
+				'start'                        => 0,
+				'length'                       => 11,
+				'expected_character_substring' => 'I am your б',
+				'expected_byte_substring'      => "I am your \xD0",
+			),
+		);
+	}
+
+	/**
+	 * @link https://github.com/php/php-src/blob/php-5.6.8/ext/mbstring/tests/mb_substr_basic.phpt
+	 */
+	public function test_mb_substr_phpcore_basic() {
+		$string_ascii = 'ABCDEF';
+		$string_mb    = base64_decode( '5pel5pys6Kqe44OG44Kt44K544OI44Gn44GZ44CCMDEyMzTvvJXvvJbvvJfvvJjvvJnjgII=' );
+
+		$this->assertSame(
+			'DEF',
+			_mb_substr( $string_ascii, 3 ),
+			'Substring does not match expected for offset 3'
+		);
+		$this->assertSame(
+			'DEF',
+			_mb_substr( $string_ascii, 3, 5, 'ISO-8859-1' ),
+			'Substring does not match expected for offset 3, length 5, with iso charset'
+		);
+
+		// Specific latin-1 as that is the default the core PHP test operates under.
+		$this->assertSame(
+			'peacrOiqng==',
+			base64_encode( _mb_substr( $string_mb, 2, 7, 'latin-1' ) ),
+			'Substring does not match expected for offset 2, length 7, with latin-1 charset'
+		);
+		$this->assertSame(
+			'6Kqe44OG44Kt44K544OI44Gn44GZ',
+			base64_encode( _mb_substr( $string_mb, 2, 7, 'utf-8' ) ),
+			'Substring does not match expected for offset 2, length 7, with utf-8 charset'
+		);
+	}
+
+	/**
+	 * @link https://github.com/php/php-src/blob/php-5.6.8/ext/mbstring/tests/mb_substr_variation1.phpt
+	 *
+	 * @dataProvider data_mb_substr_phpcore_input_type_handling
+	 *
+	 * @param mixed  $input    Input to pass to the function.
+	 * @param string $expected Expected function output.
+	 */
+	public function test_mb_substr_phpcore_input_type_handling( $input, $expected ) {
+		$start  = 0;
+		$length = 5;
+
+		$this->assertSame( $expected, _mb_substr( $input, $start, $length ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array
+	 */
+	public function data_mb_substr_phpcore_input_type_handling() {
+		$heredoc = <<<EOT
+hello world
+EOT;
+
+		return array(
+			'integer zero'                   => array(
+				'input'    => 0,
+				'expected' => '0',
+			),
+			'integer 1'                      => array(
+				'input'    => 1,
+				'expected' => '1',
+			),
+			'positive integer'               => array(
+				'input'    => 12345,
+				'expected' => '12345',
+			),
+			'negative integer'               => array(
+				'input'    => -2345,
+				'expected' => '-2345',
+			),
+			// Float data.
+			'positive float with fraction'   => array(
+				'input'    => 10.5,
+				'expected' => '10.5',
+			),
+			'negative float with fraction'   => array(
+				'input'    => -10.5,
+				'expected' => '-10.5',
+			),
+			'float scientific whole number'  => array(
+				'input'    => 12.3456789000e10,
+				'expected' => '12345',
+			),
+			'float scientific with fraction' => array(
+				'input'    => 12.3456789000E-10,
+				'expected' => '1.234',
+			),
+			'float, fraction only'           => array(
+				'input'    => .5,
+				'expected' => '0.5',
+			),
+			// Null data.
+			'null'                           => array(
+				'input'    => null,
+				'expected' => '',
+			),
+			// Boolean data.
+			'boolean true'                   => array(
+				'input'    => true,
+				'expected' => '1',
+			),
+			'boolean false'                  => array(
+				'input'    => false,
+				'expected' => '',
+			),
+			// Empty data.
+			'empty string'                   => array(
+				'input'    => '',
+				'expected' => '',
+			),
+			// String data.
+			'double quoted string'           => array(
+				'input'    => "string'",
+				'expected' => 'strin',
+			),
+			'single quoted string'           => array(
+				'input'    => 'string',
+				'expected' => 'strin',
+			),
+			'heredoc string'                 => array(
+				'input'    => $heredoc,
+				'expected' => 'hello',
+			),
+			// Object data.
+			'object with __toString method'  => array(
+				'input'    => new ClassWithToStringForMbSubstr(),
+				'expected' => 'Class',
+			),
+		);
+	}
+}
+
+/* used in data_mb_substr_phpcore_input_type_handling() */
+class ClassWithToStringForMbSubstr {
+	public function __toString() {
+		return 'Class object';
+	}
+}