Compat: Fix mb_substr() polyfill for out-of-range offsets.

The delegation to `substr()` in the `_mb_substr()` polyfill left some results returning `false` on PHP < 8.0, but `mb_substr()` always returns an empty string in these cases.

This patch updates the behavior to match `mb_substr()`. Some issues were not detected due to duplicate test names that appeared in the [60969] refactor. These have been corrected as part of this patch.

Developed in: WordPress#12302
Discussed in: https://core.trac.wordpress.org/ticket/64894

Follow-up to [60969].

Props dmsnell, soean.
See #64894.


git-svn-id: https://develop.svn.wordpress.org/trunk@62576 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell

src/wp-includes/compat.php

@@ -298,7 +298,14 @@ function _mb_substr( $str, $start, $length = null, $encoding = null ) {
 
 	// The solution below works only for UTF-8; treat all other encodings as byte streams.
 	if ( ! _is_utf8_charset( $encoding ?? get_option( 'blog_charset' ) ) ) {
-		return is_null( $length ) ? substr( $str, $start ) : substr( $str, $start, $length );
+		$result = is_null( $length ) ? substr( $str, $start ) : substr( $str, $start, $length );
+
+		/*
+		 * For an out-of-range start, substr() returns false on PHP < 8.0 but an
+		 * empty string on PHP >= 8.0. mb_substr() always returns an empty string,
+		 * so normalize to match its behavior across all supported PHP versions.
+		 */
+		return false === $result ? '' : $result;
 	}
 
 	$total_length = ( $start < 0 || $length < 0 )

tests/phpunit/tests/compat/mbSubstr.php

@@ -46,18 +46,18 @@ public function test_8bit_mb_substr( $input_string, $start, $length ) {
 	 */
 	public function data_utf8_substrings() {
 		return array(
-			'баба'           => array( 'баба', 0, 3 ),
-			'баба'           => array( 'баба', 0, -1 ),
-			'баба'           => array( 'баба', 1, null ),
-			'баба'           => array( 'баба', -3, null ),
-			'баба'           => array( 'баба', -3, 2 ),
-			'баба'           => array( 'баба', -2, 1 ),
-			'баба'           => array( 'баба', 30, 1 ),
-			'баба'           => array( 'баба', 15, -30 ),
-			'баба'           => array( 'баба', -5, -5 ),
-			'баба'           => array( 'баба', 5, -3 ),
-			'баба'           => array( 'баба', -3, 5 ),
-			'I am your баба' => array( 'I am your баба', 0, 11 ),
+			'positive start, positive length'              => array( 'баба', 0, 3 ),
+			'positive start, negative length'              => array( 'баба', 0, -1 ),
+			'positive start, null length'                  => array( 'баба', 1, null ),
+			'negative start, null length'                  => array( 'баба', -3, null ),
+			'negative start, positive length'              => array( 'баба', -3, 2 ),
+			'negative start near end, positive length'     => array( 'баба', -2, 1 ),
+			'start beyond length, positive length'         => array( 'баба', 30, 1 ),
+			'start beyond length, large negative length'   => array( 'баба', 15, -30 ),
+			'negative start beyond start, negative length' => array( 'баба', -5, -5 ),
+			'start beyond length, negative length'         => array( 'баба', 5, -3 ),
+			'negative start, length beyond end'            => array( 'баба', -3, 5 ),
+			'multibyte character in longer string'         => array( 'I am your баба', 0, 11 ),
 		);
 	}