Upgrade/Install: Improve `wp_opcache_invalidate()` performance.

Changes from using `preg_match()` based `.php` extension checking to using `substr()` in `wp_opcache_invalidate()`.

Props jnylen0, aaroncampbell, SergeyBiryukov, desrosj, mikeschroder.
Fixes #50784.

git-svn-id: https://develop.svn.wordpress.org/trunk@48657 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/file.php

@@ -2356,7 +2356,7 @@ function wp_opcache_invalidate( $filepath, $force = false ) {
 	}
 
 	// Verify that file to be invalidated has a PHP extension.
-	if ( ! preg_match( '/\.(?:php)$/i', $filepath ) ) {
+	if ( '.php' !== strtolower( substr( $filepath, -4 ) ) ) {
 		return false;
 	}