PHP：Imagecopy和Imagecopymerge不兼容两个png

I would put a logo image in another image, the two images are png, I try both functions imagecopy and imagecopymerge but nothing to do, I do not see the result when I open the image on the disk, however both functions returns true. Someone can help me, here is the code I use:

     $data = substr($_POST['imageData'], strpos($_POST['imageData'], ",") + 1);
     $decodedData = base64_decode($data);
     $img = "assets\\images\\simulation\\user\\img777.png";
     $srcpath =  "assets\\images\\logo.png";
     fp = fopen($img , 'wb');
     fwrite($fp, $decodedData);
     fclose($fp);//work until this place
     $dest =imagecreatefrompng($img);
     $src =imagecreatefrompng($srcpath);
     imagecopy ($dest,$src, 0, 0, 0, 0, imagesx($src),imagesy($src));

     // or imagecopymerge ($dest,$src, 0, 0, 0, 0, imagesx($src),imagesy($src), $pct); with $pct =0 or 50

     /*or i have also try :  $cut = imagecreatetruecolor(imagesx($src),imagesy($src));
        imagecopy($cut, $dest, 0, 0, 0, 0, imagesx($src),imagesy($src));
        imagecopy($cut, $src, 0, 0, 0, 0, imagesx($src),imagesy($src));
        imagecopymerge ($dest,$cut, 0, 0, 0, 0, imagesx($src),imagesy($src), $pct); with $pct =0 or 50*/

here are a example for the image logo.png : http://upload.wikimedia.org/wikipedia/commons/4/48/EBay_logo.png

Here are the other image.png : http://www.noelshack.com/2014-40-1412264764-img68634dxn777.png

the result it's the same that image.png ...

can you help me, thanks.

I solved this problem with the following code:

    imagealphablending($dest, true);
    imagealphablending($src, true);
    imagesavealpha($dest, true);
    imagesavealpha($src, true);
    imagecopy ($dest,$src, 5, imagesy($dest) - (imagesy($src)+5), 0, 0,imagesx($src),imagesy($src));
    imagealphablending($dest, true);
    imagesavealpha($dest, true);
    imagepng($dest,$img);
    imagedestroy($dest);
    imagedestroy($src);

in place of :

    $dest =imagecreatefrompng($img);
    $src =imagecreatefrompng($srcpath);
    imagecopy ($dest,$src, 0, 0, 0, 0, imagesx($src),imagesy($src));