<?php
$str = 'callback({"client_id":"YOUR_APPID","openid":"YOUR_OPENID"});';
$lpos = strpos($str, "(");
$rpos = strrpos($str, ")");
$str = substr($str, $lpos+1, $rpos-$lpos-1);
$json = json_decode($str);
// print_r($json);
$openid = $json->openid;
echo $openid;