(PHP 4 >= 4.0.1, PHP 5, PHP 7)
imagecopymerge — Copy and merge part of an image
$dst_im
   , resource $src_im
   , int $dst_x
   , int $dst_y
   , int $src_x
   , int $src_y
   , int $src_w
   , int $src_h
   , int $pct
   )
   Copy a part of src_im onto
   dst_im starting at the x,y coordinates
   src_x, src_y  with
   a width of src_w and a height of
   src_h.  The portion defined will be copied
   onto the x,y coordinates, dst_x and
   dst_y.
  
dst_imDestination image link resource.
src_imSource image link resource.
dst_xx-coordinate of destination point.
dst_yy-coordinate of destination point.
src_xx-coordinate of source point.
src_yy-coordinate of source point.
src_wSource width.
src_hSource height.
pct
       The two images will be merged according to pct
       which can range from 0 to 100.  When pct = 0,
       no action is taken, when 100 this function behaves identically
       to imagecopy() for pallete images, except for
       ignoring alpha components, while it implements alpha transparency
       for true colour images.
      
   Returns TRUE on success or FALSE on failure.
  
Example #1 Merging two copies of the PHP.net logo with 75% transparency
<?php
// Create image instances
$dest = imagecreatefromgif('php.gif');
$src = imagecreatefromgif('php.gif');
// Copy and merge
imagecopymerge($dest, $src, 10, 10, 0, 0, 100, 47, 75);
// Output and free from memory
header('Content-Type: image/gif');
imagegif($dest);
imagedestroy($dest);
imagedestroy($src);
?>