<?php
$str = "123456789abcdefgh";
preg_match("#abcd#", $str, $matches, PREG_OFFSET_CAPTURE
); $index = $matches;
PD9waHAKCiRzdHIgPSAiMTIzNDU2Nzg5YWJjZGVmZ2giOwpwcmVnX21hdGNoKCIjYWJjZCMiLCAkc3RyLCAkbWF0Y2hlcywgUFJFR19PRkZTRVRfQ0FQVFVSRSk7CiRpbmRleCA9ICRtYXRjaGVzOwoKcHJpbnRfcigkaW5kZXgpOwo=