gistfile1.txt

<?php

$txtOrig = $txt = file_get_contents('bad.txt');

function test($str) {
        preg_replace_callback("/test/u", function($matches) { return ''; }, $str);
        return preg_last_error() === 4 /* PREG_BAD_UTF8_ERROR */;
}

$i = 0;
while ( strlen ( $txt ) > 0 ) {
        $txt = substr($txt, 1);
        if (test($txt) === false) {
                break;
        }
        $i++;   
}

echo substr($txtOrig, $i - 5, 15);