PHP File Upload

file_upload.php

<?php

ini_set('display_errors', 1);
error_reporting(E_ALL);

define('UPLOAD_DIRECTORY', 'uploads/'); // directory path for the uploads
define('UPLOAD_HASH',       md5(time()));

if (isset($_FILES['file']['name']))
{
    if ($_FILES['file']['error'] == UPLOAD_ERR_OK)
    {
        // Processes your file here
        $upload_dir = $_SERVER['DOCUMENT_ROOT'].'/'.UPLOAD_DIRECTORY;

        if (is_dir($upload_dir) && is_writable($upload_dir))
        {
            // upload logic
            if (move_uploaded_file($_FILES['file']['tmp_name'], $upload_dir.UPLOAD_HASH.'-'.$_FILES['file']['name']))
            {
                echo json_encode(array
                (
                    'success' => true,
                    'file'    => array('short'=> $_FILES['file']['name'], 'full' => UPLOAD_HASH.'-'.$_FILES['file']['name'])
                ));
            }
            else
            {
                echo json_encode(array
                (
                    'error'   => true,
                    'message' => 'Error moving temporary file to upload directory.'
                ));
            }

            // die("Cannot write to destination file");
        }
        else
        {
            echo json_encode(array
            (
                'error'   => true,
                'message' => 'Upload directory is not writable, or does not exist.'
            ));
        }
    }
    else
    {
        switch ($_FILES['file']['error'])
        {
            case UPLOAD_ERR_INI_SIZE:
                $message = 'The uploaded file exceeds the upload_max_filesize directive in php.ini';
                break;
            case UPLOAD_ERR_FORM_SIZE:
                $message = 'The uploaded file exceeds the MAX_FILE_SIZE directive that was specified in the HTML form.';
                break;
            case UPLOAD_ERR_PARTIAL:
                $message = 'The uploaded file was only partially uploaded.';
                break;
            case UPLOAD_ERR_NO_FILE:
                $message = 'No file was uploaded.';
                break;
            case UPLOAD_ERR_NO_TMP_DIR:
                $message = 'Missing a temporary folder.';
                break;
            case UPLOAD_ERR_CANT_WRITE:
                $message= 'Failed to write file to disk.';
                break;
            case  UPLOAD_ERR_EXTENSION:
                $message = 'File upload stopped by a PHP extension';
                break;
            default: $message = 'Unknown upload error.';
            break;
        }

        echo json_encode(array
        (
            'error'   => true,
            'message' => $message
        ));
    }
}
else
{
    echo json_encode(array
    (
        'error'   => true,
        'message' => 'No file was uploaded'
    ));
}