MySQL | INSERT INTO funktioniert nicht

<?php
session_start();


function randomstring($length = 10) {
  return substr(str_shuffle(str_repeat('ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789', 10)), 0, 5);
}


	include("dbconnect.php");
    $connection = mysql_select_db($mysqldatabase);


if(isset($_FILES['file']['name']) && $connection) {
    $pi = pathinfo($_FILES['file']['name']);
    if(strtolower($pi['extension']) == 'bmp' || strtolower($pi['extension']) == 'gif' || strtolower($pi['extension']) == 'jpg' || strtolower($pi['extension']) == 'jpeg' || strtolower($pi['extension']) == 'png' || strtolower($pi['extension']) == 'tif') {
        if($_FILES['file']['size'] < 8388608) {
            $filename = randomstring() . '.' . strtolower($pi['extension']);
            $key = md5(md5(md5(md5(md5(base64_encode(base64_encode($filename)))))));
            if(move_uploaded_file($_FILES['file']['tmp_name'], 'files/' . $filename)) {
                //file_put_contents('files/' . $filename . '.key', $key);
                $timestamp = time();
                if(isset($_SESSION['username']))
                {
                    $sql = "INSERT INTO `images` (`filename`, `key`, `timestamp`, `uploader`, `uploaderip`) VALUES ('" . mysql_real_escape_string($filename) . "', '" . mysql_real_escape_string($key) . "', '" . mysql_real_escape_string($timestamp)  . "', '" . mysql_real_escape_string($_SESSION['username']) . "', '" . mysql_real_escape_string($_SERVER['REMOTE_ADDR']) . "');";
                }
                else
                {
                    $sql = "INSERT INTO `images` (`filename`, `key`, `timestamp`, `uploader`, `uploaderip`) VALUES ('" . mysql_real_escape_string($filename) . "', '" . mysql_real_escape_string($key) . "', '" . mysql_real_escape_string($timestamp)  . "', 'Kein Uploader', '" . mysql_real_escape_string($_SERVER['REMOTE_ADDR']) . "');";
                }
	            mysql_query($sql);
                header('Location: result.php?code=4&filename=' . $filename . '&key=' . $key);
                exit();
            } else {
                header('Location: result.php?code=0');
                exit();
            }
        } else {
            header('Location: result.php?code=1');
            exit();
        }
    } else {
        header('Location: result.php?code=2');
        exit();
    }
} else {
    header('Location: result.php?code=3');
    exit();
}
?>