ok ho risolto cosi :

codice:
<?
if ($_POST[md5]) {
	$fd = fopen("md5.txt", "r");
	while (!feof($fd)) {
		$buffer = fgets($fd, 4096);
		$buffer = explode(":", $buffer);
		$buffer[0] = trim($buffer[0]);
		if (strtolower($buffer[0]) == strtolower($_POST[md5])) {
			echo trim($buffer[1])."\n";
			fclose($fd);
			die();
		}
	}
	fclose($fd);
}
?>
Grazie !