Unable to water mark images with php - php

Question

I was unable to watermark my images. I used the php manual found at this link
http://www.php.net/manual/en/image.examples-watermark.php
And also tried the sitepoint tutorial here
http://www.sitepoint.com/watermark-images-php/
but getting the same error that the image cannot be displayed because its have some errors. can somebody let me know whats wrong with the code or suggest me some better solution.
My code is here :
header('content-type: image/jpeg');
$watermark = imagecreatefrompng('wm.png');
$watermark_width = imagesx($watermark);
$watermark_height = imagesy($watermark);
$image = imagecreatetruecolor($watermark_width, $watermark_height);
$image = imagecreatefromjpeg('img.jpg');
$size = getimagesize('img.jpg');
$dest_x = $size[0] - $watermark_width - 5;
$dest_y = $size[1] - $watermark_height - 5;
imagecopymerge($image, $watermark, $dest_x, $dest_y, 0, 0, $watermark_width, $watermark_height, 100);
imagejpeg($image);
imagedestroy($image);
imagedestroy($watermark);

Answer 1

Check all the return values of the functions you call, e.g.
$watermark = imagecreatefrompng('wm.png');
if ( !$watermark ) {
die('Sorry ' . __LINE__); // you might want to use something else here - just an example....
}
and then set the content type to image/jpeg not before you're actually trying to send the image
if ( headers_sent($file, $line) ) {
die('oops '.__LINE__);
}
else {
header('content-type: image/jpeg');
imagejpeg($image);
}
...makes it easier to pin-point the error.