diff --git a/src/wp-includes/compat.php b/src/wp-includes/compat.php
index 7377a384c8..77762cb6f6 100644
--- a/src/wp-includes/compat.php
+++ b/src/wp-includes/compat.php
@@ -505,3 +505,21 @@ endif;
 if ( ! function_exists( 'spl_autoload_register' ) ) {
 	require_once ABSPATH . WPINC . '/spl-autoload-compat.php';
 }
+
+if ( ! function_exists( 'is_countable' ) ) {
+	/**
+	 * Polyfill for is_countable() function added in PHP 7.3.
+	 *
+	 * Verify that the content of a variable is an array or an object
+	 * implementing Countable.
+	 *
+	 * @since 4.9.6
+	 *
+	 * @param mixed $var The value to check.
+	 *
+	 * @return bool True if `$var` is countable, false otherwise.
+	 */
+	function is_countable( $var ) {
+		return ( is_array( $var ) || $var instanceof Countable );
+	}
+}
diff --git a/tests/phpunit/tests/compat.php b/tests/phpunit/tests/compat.php
index 3d873745b9..43f1600f25 100644
--- a/tests/phpunit/tests/compat.php
+++ b/tests/phpunit/tests/compat.php
@@ -186,6 +186,50 @@ EOT;
 		$this->assertEquals( '["foo"]', $json->encodeUnsafe( array( 'foo' ) ) );
 		$this->assertEquals( array( 'foo' ), $json->decode( '["foo"]' ) );
 	}
+
+	/**
+	 * @ticket 43583
+	 */
+	function test_is_countable_availability() {
+		$this->assertTrue( function_exists( 'is_countable' ) );
+	}
+
+	/**
+	 * Test is_countable() polyfill.
+	 *
+	 * @ticket 43583
+	 *
+	 * @dataProvider countable_variable_test_data
+	 */
+	function test_is_countable_functionality( $variable, $is_countable ) {
+		$this->assertEquals( 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 ),
+		);
+	}
 }
 
 /* used in test_mb_substr_phpcore */
@@ -194,3 +238,12 @@ class classA {
 		return 'Class A object';
 	}
 }
+
+class ArrayIteratorFake extends ArrayIterator {
+}
+
+class CountableFake implements Countable {
+	public function count() {
+		return 16;
+	}
+}