parse.php

<?php
$str = "Sending: 6011 1234 4321 1234:12/2016:543.21:John Doe
Received: Mon Apr 27 18:07:22 CDT 2015: Discover: 6011 1234 4321 1234 is valid: authorization code: 472080 for $543.21";

$a = preg_split("/\n/", $str);
array_shift($a);
$c = preg_match_all('/authorization\scode:\s[0-9]+/', $a[0], $matches);
$d = preg_split('/\n/', $matches[0][0]);
$e = explode(" ", $d[0]);
var_dump($e[2]);