- preg_replace: documentation ( source)
<?php
$source = '/<img src="http:\/\/website\.com\/char\/sign\/(.+?)\.png">/';
$new = '[sign]$1[/sign]';
$text = '<img src="http://website.com/char/sign/Name+Surname.png">, <img src="http://website.com/char/sign/Bob-Robinson.png">, <img src="http://website.com/char/sign/Michael%2527Ross.png">';
$text = preg_replace($source, $new, $text);
echo $text;