- str_replace: documentation ( source)
<?php
$pidsite=1234;
$script="<!-- LeadFormix --><script type='text/javascript'>bf_action_name='';bf_idsite=8635;urlid=;bf_url='https://vlog.leadformix.com/bf/bf.php';(function(){var lfh=document.createElement('script');lfh.type='text/javascript';lfh.async=true;lfh.src='https://vlog.leadformix.com/bf/lfx.js';var s=document.getElementsByTagName('head')[0];s.appendChild(lfh);})();</script><noscript><p>Market Automation<img src='https://vlog.leadformix.com/bf/bf.php' style='border:0' alt='Marketing Automation Platform'/></p></noscript><!-- /LeadFormix -->";
echo $script = str_replace(";bf_idsite=8635;", ";bf_idsite=1234;", $script);
?>