3v4l.org

run code in 500+ PHP versions simultaneously
<?php $search = 'The'; $replace = 'A'; $subject = "The Quick Brown Fox Jumps Over The Lazy Dog's Thermos!"; echo preg_replace( '/.*\K\b' . preg_quote($search, '/') . '\b/i', $replace, $subject );

preferences:
52.38 ms | 1523 KiB | 5 Q