- preg_replace: documentation ( source)
<?php
$embedCode = "<iframe width='800' height='600'> </iframe>";
echo preg_replace('~(<iframe[^>]*\swidth=["\']?)(\d+)(["\']?\s?.*)~', '${1}' . '666' . '${3}', $embedCode);
echo preg_replace('~(<iframe[^>]*\sheight=["\']?)(\d+)(["\']?\s?.*)~', '${1}' . '666' . '${3}', $embedCode);