Add text to image PHP GD

Question

I want to store a copy of uploaded image (with watermark) in different catalog. First I copy image to that catalog and then I add a watermark with imagegettftext function but I get fottowing error and image is unchanged.
Warning: imagettftext() expects parameter 1 to be resource, string given in /var/www/dev/web/gallery.php on line 48
The path given in argument is correct, what am I doing wrong? Here's my code:

$image_new_name=uniqid('',true).'.'.$image_ext;
$image_dest='images/'.$image_new_name;

if (move_uploaded_file($image_tmp_name,$image_dest)) {

    $image_watermark="images/watermark/".$image_new_name;
    if (copy($image_dest, $image_watermark)) {
    $watermark_color=imagecreate(10,10);
    $white=imagecolorallocate($watermark_color,255,255,255);
    $font='arial.ttf';
    imagettftext($image_watermark, 15, 45, 0,0,$white,$font,$watermark);
    }