- var_dump: documentation ( source)
- preg_match: documentation ( source)
<?php
$txt ='<!-- [newsletter:123123] -->
<html style="font-size:14px;font:inherit;vertical-align:baseline;font-family:Open Sans,sans-serif;height:100%;">
<head><meta http-eq';
preg_match('/newsletter:(.*?)/', $txt, $matches);
var_dump($matches);