diff --git a/src/wp-includes/compat.php b/src/wp-includes/compat.php
index b95275455c..2a8664c065 100644
--- a/src/wp-includes/compat.php
+++ b/src/wp-includes/compat.php
@@ -375,6 +375,48 @@ if ( ! function_exists( 'is_iterable' ) ) {
 	}
 }
 
+if ( ! function_exists( 'array_key_first' ) ) {
+	/**
+	 * Polyfill for array_key_first() function added in PHP 7.3.
+	 *
+	 * Get the first key of the given array without affecting
+	 * the internal array pointer.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param array $arr An array.
+	 * @return string|int|null The first key of array if the array
+	 *                         is not empty; `null` otherwise.
+	 */
+	function array_key_first( array $arr ) {
+		foreach ( $arr as $key => $value ) {
+			return $key;
+		}
+	}
+}
+
+if ( ! function_exists( 'array_key_last' ) ) {
+	/**
+	 * Polyfill for `array_key_last()` function added in PHP 7.3.
+	 *
+	 * Get the last key of the given array without affecting the
+	 * internal array pointer.
+	 *
+	 * @since 5.9.0
+	 *
+	 * @param array $arr An array.
+	 * @return string|int|null The last key of array if the array
+	 *.                        is not empty; `null` otherwise.
+	 */
+	function array_key_last( array $arr ) {
+		if ( empty( $arr ) ) {
+			return null;
+		}
+		end( $arr );
+		return key( $arr );
+	}
+}
+
 // IMAGETYPE_WEBP constant is only defined in PHP 7.1 or later.
 if ( ! defined( 'IMAGETYPE_WEBP' ) ) {
 	define( 'IMAGETYPE_WEBP', 18 );
diff --git a/tests/phpunit/tests/compat/arrayKeyFirst.php b/tests/phpunit/tests/compat/arrayKeyFirst.php
new file mode 100644
index 0000000000..613845aba0
--- /dev/null
+++ b/tests/phpunit/tests/compat/arrayKeyFirst.php
@@ -0,0 +1,76 @@
+<?php
+
+/**
+ * @group compat
+ *
+ * @covers ::array_key_first
+ */
+class Tests_Compat_arrayKeyFirst extends WP_UnitTestCase {
+
+	/**
+	 * Test that array_key_first() is always available (either from PHP or WP).
+	 * @ticket 45055
+	 */
+	public function test_array_key_first_availability() {
+		$this->assertTrue( function_exists( 'array_key_first' ) );
+	}
+
+	/**
+	 * @dataProvider data_array_key_first
+	 *
+	 * @ticket 45055
+	 *
+	 * @param bool $expected The value of the key extracted to extracted from given array.
+	 * @param array $arr     The array to get first key from.
+	 */
+	public function test_array_key_first( $expected, $arr ) {
+		if ( ! function_exists( 'array_key_first' ) ) {
+			$this->markTestSkipped( 'array_key_first() is not available.' );
+		} else {
+			$this->assertSame(
+				$expected,
+				array_key_first( $arr )
+			);
+		}
+
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_array_key_first() {
+		return array(
+			'string key'  => array(
+				'expected' => 'key1',
+				'arr'      => array(
+					'key1' => 'val1',
+					'key2' => 'val2',
+				),
+			),
+			'int key'     => array(
+				'expected' => 99,
+				'arr'      => array(
+					99 => 'val1',
+					1  => 'val2',
+				),
+			),
+			'no key'      => array(
+				'expected' => 0,
+				'arr'      => array( 'val1', 'val2' ),
+			),
+			'multi array' => array(
+				'expected' => 99,
+				'arr'      => array(
+					99 => array( 22 => 'val1' ),
+					1  => 'val2',
+				),
+			),
+			'empty array' => array(
+				'expected' => null,
+				'arr'      => array(),
+			),
+		);
+	}
+}
diff --git a/tests/phpunit/tests/compat/arrayKeyLast.php b/tests/phpunit/tests/compat/arrayKeyLast.php
new file mode 100644
index 0000000000..77510c8320
--- /dev/null
+++ b/tests/phpunit/tests/compat/arrayKeyLast.php
@@ -0,0 +1,81 @@
+<?php
+
+/**
+ * @group compat
+ *
+ * @covers ::array_key_last
+ */
+class Tests_Compat_ArrayKeyLast extends WP_UnitTestCase {
+
+	/**
+	 * Test that array_key_last() is always available (either from PHP or WP).
+	 *
+	 * @ticket 45055
+	 */
+	public function test_array_key_last_availability() {
+		$this->assertTrue( function_exists( 'array_key_last' ) );
+	}
+
+	/**
+	 * @dataProvider data_array_key_last
+	 *
+	 * @ticket 45055
+	 *
+	 * @param bool  $expected The value of the key extracted to extracted from given array.
+	 * @param array $arr      The array to get last key from.
+	 */
+	public function test_array_key_last( $expected, $arr ) {
+		if ( ! function_exists( 'array_key_last' ) ) {
+			$this->markTestSkipped( 'array_key_last() is not available.' );
+		} else {
+			$this->assertSame( $expected, array_key_last( $arr ) );
+		}
+	}
+
+	/**
+	 * Data provider for test_array_key_last().
+	 *
+	 * @return array
+	 */
+	public function data_array_key_last() {
+		return array(
+			'string key'  => array(
+				'expected' => 'key2',
+				'arr'      => array(
+					'key1' => 'val1',
+					'key2' => 'val2',
+				),
+			),
+			'int key'     => array(
+				'expected' => 1,
+				'arr'      => array(
+					99 => 'val1',
+					1  => 'val2',
+				),
+			),
+			'no key'      => array(
+				'expected' => 1,
+				'arr'      => array( 'val1', 'val2' ),
+			),
+			'multi array' => array(
+				'expected' => 1,
+				'arr'      => array(
+					99 => array( 22 => 'val1' ),
+					1  => 'val2',
+				),
+			),
+			'mixed keys'  => array(
+				'expected' => 1,
+				'arr'      => array(
+					'val1',
+					'key2' => 'val2',
+					'val3',
+				),
+			),
+			'empty array' => array(
+				'expected' => null,
+				'arr'      => array(),
+			),
+		);
+	}
+}