INSERT INTO called from a php script fails

I am having an ios app and I am calling a script from my online server, in order to insert a value into a table.

In this code:

//try to register the user
$result = query("INSERT INTO login(username, pass) VALUES('%s','%s')", $user, $pass);
if (!$result['error']) {
    //success
    login($user, $pass);
} else {
    //error
    errorJson($result['error']);
}

I always get into the else clause and it is printed Database error.

Note: errorJson is a function that makes a json output from a string in order to send it back to the iphone app.

A normal select from where plays normally, so there is no chance that I am not connected to the DB.

The whole code is here:

function register($user, $pass) {
    //check if username exists
    $login = query("SELECT username FROM login WHERE username='%s' limit 1", $user);
    if (count($login['result'])>0) {
        errorJson('Username already exists');
    }
       //try to register the user
       $result = query("INSERT INTO login(username, pass) VALUES('%s','%s')", $user, $pass); 
        if (! $result['error']) {
        //success
        login($user, $pass);
        } else {
    //error
              errorJson($result['error']);
        }
}

If the username exists the NSLog above is printed normally, so I guess I can connect to the DB.

and this is my query function:

//executes a given sql query with the params and returns an array as result
function query() {
    global $link;
    $debug = false;

    //get the sql query
    $args = func_get_args();
    $sql = array_shift($args);

    //secure the input
    for ($i=0;$i<count($args);$i++) {
        $args[$i] = urldecode($args[$i]);
        $args[$i] = mysql_real_escape_string($link, $args[$i]);
    }

    //build the final query
    $sql = vsprintf($sql, $args);

    if ($debug) print $sql;

    //execute and fetch the results
    $result = mysql_query($link, $sql);
    if (mysql_errno($link)==0 && $result) {

        $rows = array();

        if ($result!==true)
        while ($d = mysql_fetch_assoc($result)) {
            array_push($rows,$d);
        }

        //return json
        return array('result'=>$rows);

    } else {

        //error
        return array('error'=>'Database error');
    }
}