<?php
$var = 'itemid=537600240%3fusername%3d& ';
$regex = '/itemid=([0-9]+)/';
echo $result[1];
?>
PD9waHAKCgkkdmFyID0gJ2l0ZW1pZD01Mzc2MDAyNDAlM2Z1c2VybmFtZSUzZCYgJzsKCQoJJHJlZ2V4ID0gJy9pdGVtaWQ9KFswLTldKykvJzsKCQoJcHJlZ19tYXRjaCAoICRyZWdleCwgJHZhciwgJHJlc3VsdCk7CgoJZWNobyAkcmVzdWx0WzFdOwoKPz4K