<?php

$dbhost = 'localhost';
$dbuser = 'root';
$dbpass = 'password';

$sql = mysql_connect($dbhost, $dbuser, $dbpass);

$dbname = 'petstore';
mysql_select_db($dbname);

$lic = $_GET['lic'];
$sql = "SELECT something FROM somewhere WHERE lic='".mysql_real_escape_string($lic);
$query = mysql_query($sql);

if(mysql_num_rows($query)<1) {

    die("Invalid!");

}

else{

 //Anything else you want it to do if it returns not false.

}

?>