diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index dd908dfcf7..84d032dd95 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -236,44 +236,6 @@ class Tests_Functions extends WP_UnitTestCase {
 		$this->assertFalse( is_serialized( 'C:16:"Serialized_Class":6:{a:0:{}}' ) );
 	}
 
-	/**
-	 * @dataProvider data_is_serialized_string
-	 */
-	public function test_is_serialized_string( $value, $result ) {
-		$this->assertSame( is_serialized_string( $value ), $result );
-	}
-
-	public function data_is_serialized_string() {
-		return array(
-			// Not a string.
-			array( 0, false ),
-
-			// Too short when trimmed.
-			array( 's:3   ', false ),
-
-			// Too short.
-			array( 's:3', false ),
-
-			// No colon in second position.
-			array( 's!3:"foo";', false ),
-
-			// No trailing semicolon.
-			array( 's:3:"foo"', false ),
-
-			// Wrong type.
-			array( 'a:3:"foo";', false ),
-
-			// No closing quote.
-			array( 'a:3:"foo;', false ),
-
-			// Wrong number of characters is close enough for is_serialized_string().
-			array( 's:12:"foo";', true ),
-
-			// Okay.
-			array( 's:3:"foo";', true ),
-
-		);
-	}
 
 	/**
 	 * @group add_query_arg
diff --git a/tests/phpunit/tests/functions/isSerializedString.php b/tests/phpunit/tests/functions/isSerializedString.php
new file mode 100644
index 0000000000..292765aab4
--- /dev/null
+++ b/tests/phpunit/tests/functions/isSerializedString.php
@@ -0,0 +1,68 @@
+<?php
+
+/**
+ * Tests for `is_serialized_string()`.
+ *
+ * @group functions.php
+ * @ticket 42870
+ */
+class Tests_Functions_Is_Serialized_String extends WP_UnitTestCase {
+
+	/**
+	 * Data provider method for testing `is_serialized_string()`.
+	 *
+	 * @return array
+	 */
+	public function _is_serialized_string() {
+		return array(
+
+			// pass array.
+			array( array(), false ),
+
+			// pass a class.
+			array( new stdClass(), false ),
+
+			// Not a string.
+			array( 0, false ),
+
+			// Too short when trimmed.
+			array( 's:3       ', false ),
+
+			// Too short.
+			array( 's:3', false ),
+
+			// No colon in second position.
+			array( 's!3:"foo";', false ),
+
+			// No trailing semicolon.
+			array( 's:3:"foo"', false ),
+
+			// Wrong type.
+			array( 'a:3:"foo";', false ),
+
+			// No closing quote.
+			array( 'a:3:"foo;', false ),
+
+			// have to use double Quotes.
+			array( "s:12:'foo';", false ),
+
+			// Wrong number of characters is close enough for is_serialized_string().
+			array( 's:12:"foo";', true ),
+
+			// Okay.
+			array( 's:3:"foo";', true ),
+		);
+	}
+
+	/**
+	 * Run tests on `is_serialized_string()`.
+	 *
+	 * @dataProvider _is_serialized_string
+	 *
+	 * @param array|object|int|string $data     Data value to test.
+	 * @param bool                    $expected Expected function result.
+	 */
+	public function test_is_serialized_string( $data, $expected ) {
+		$this->assertSame( $expected, is_serialized_string( $data ) );
+	}
+}