- preg_replace: documentation ( source)
<?php
$string = 'foobarfoobarfoobarfoobar<center><a href="ciao"><img alt="" src="image.png" /></a></center>foobarfoobarfoobar!';
$image = preg_replace("/.*\<center\>\<a href=\"(.*)\"\>\<img alt=\"\" src=\"(.*)\" \/\>\<\/a\>\<\/center\>.*/", "$2", $string);
echo $image;