<?php
$str='[zx1][zx21]...';
$search = '/\[zx(.+)\]/U';
$replace = '<img src="$1.gif">';
echo htmlspecialchars(preg_replace($search,$replace,$str));
?>
$str='[zx1][zx21]...';
$search = '/\[zx(.+)\]/U';
$replace = '<img src="$1.gif">';
echo htmlspecialchars(preg_replace($search,$replace,$str));
?>
echo preg_replace("/\[zx(\d+).*/","<img src='\\1.gif'>",$str);