Cara supaya dapetin $flag ?

Cara supaya dapetin $flag ?
<?php
show_source(__FILE__);
function length($x) {
if (is_array($x))
return count($x);
return strlen($x);

}

require 'flag.php';

$pass = $_GET['key'];

$ok = length($pass)==4;

for ($i=0; $i if (strcmp($pass[$i], $password[$i])!=0) {
$ok = false;
echo "invalid $i";
break;
}
}

if ($ok) {
echo "The Flag is: ".$flag;
exit(0);
} else {
if (isset($_GET['key']))
echo "Invalid";
}