<?php

// Hier mysql_connect, mysql_select_db, am besten includen

if(isset($_GET["username"], $_GET["password"])
    && !empty($_GET["username"])
    && !empty($_GET["password"]))
{
    $username = mysql_real_escape_string($_GET["username"]);
    $password = mysql_real_escape_String($_GET["password"]);
    // ^- ggf. ersetzen durch z.B. -> $password = md5($_GET["password"]);

    $query = mysql_query("SELECT id, username, gang, rank, isleader FROM users WHERE username='" . $username . "' AND password='" . $password . "'"); // or die(mysql_error());

    if(mysql_num_rows($query) > 0)
    {
        $assoc = mysql_fetch_assoc($query);
        echo json_encode($assoc);
    } else {
        echo json_encode(array("id"=>-1));
    }
}