How do I combine an image and save it to the site directory?

Question

PHP

Good afternoon, I need to implement combining images and saving to the site directory, but something doesn’t work for me.
src - source image, blue rectangle
srcUpload - image to be overlaid, black cross

$posX = $_POST['podp_x'];
  $posY = $_POST['podp_y'];
  $dest = imagecreatefromjpeg('../'.$_POST['src']);
  $src = imagecreatefrompng('../'.$_POST['srcUpload']);
  echo $_POST['src'];
  echo $_POST['srcUpload'];

  imagealphablending($dest, false);
  imagesavealpha($dest, true);
  imagecopymerge($src, $dest, 0, 0, $posX, $posY, 445, 492, 100);
  
  $name = 'test.png';
  imagefilter($dest, IMG_FILTER_GRAYSCALE);
  imagepng( $dest, "../upload/".$name);

Saves the picture, but does not superimpose another one on it, and at the same time the rectangle becomes black.