<?php
$str = 'window.location.href="http://m...content-available-to-author-only...e.com";';
preg_match('/window\.location\.href="(.*?)";/', $str, $result); echo $result[1];
PD9waHAKCiRzdHIgPSAnd2luZG93LmxvY2F0aW9uLmhyZWY9Imh0dHA6Ly9tLi4uY29udGVudC1hdmFpbGFibGUtdG8tYXV0aG9yLW9ubHkuLi5lLmNvbSI7JzsKcHJlZ19tYXRjaCgnL3dpbmRvd1wubG9jYXRpb25cLmhyZWY9IiguKj8pIjsvJywgJHN0ciwgJHJlc3VsdCk7CmVjaG8gJHJlc3VsdFsxXTs=