How can I debug problems with move_uploaded_file?

I have a form like

<form action="send.php" method="post" enctype="multipart/form-data">
    <div>
        <label for="subject">Subject</label>
        <input type="text" name="subject" />
    </div>
    <div>
        <label for="image">Image</label>
        <input type="file" name="image" />
    </div>
    <input type="submit" value="Send" />
</form>

PHP like

echo '<pre>'; print_r($_FILES); echo '</pre>';
if (move_uploaded_file($_FILES['image']['tmp_name'], 'images/' . $_FILES['image']['name'])) {
    echo 'ok';
} else {
    echo 'error!';
};

I keep getting error the print_r looks like

Array
(
    [image] => Array
        (
            [name] => Untitled-1.jpg
            [type] => image/jpeg
            [tmp_name] => /tmp/phpprWdjN
            [error] => 0
            [size] => 61768
        )

)