- preg_quote: documentation ( source)
- preg_replace: documentation ( source)
<?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
);