Chuẩn bị 2 ảnh, 1 ảnh gốc(main.jpg) và một ảnh dùng để đóng dấu(watermark.png).

Tạo thư mục watermark và chép 2 ảnh trên vào.

Lưu đoạn mã sau thành file watermark.php vào thư mục watermark

<?php
class watermark{

	# given two images, return a blended watermarked image
	function create_watermark( $main_img_obj, $watermark_img_obj, $alpha_level = 100 ) {
		$alpha_level	/= 100;	# convert 0-100 (%) alpha to decimal
	
		# calculate our images dimensions
		$main_img_obj_w	= imagesx( $main_img_obj );
		$main_img_obj_h	= imagesy( $main_img_obj );
		$watermark_img_obj_w	= imagesx( $watermark_img_obj );
		$watermark_img_obj_h	= imagesy( $watermark_img_obj );
		
		# determine center position coordinates
		$main_img_obj_min_x	= floor( ( $main_img_obj_w / 2 ) - ( $watermark_img_obj_w / 2 ) );
		$main_img_obj_max_x	= ceil( ( $main_img_obj_w / 2 ) + ( $watermark_img_obj_w / 2 ) );
		$main_img_obj_min_y	= floor( ( $main_img_obj_h / 2 ) - ( $watermark_img_obj_h / 2 ) );
		$main_img_obj_max_y	= ceil( ( $main_img_obj_h / 2 ) + ( $watermark_img_obj_h / 2 ) ); 
		
		# create new image to hold merged changes
		$return_img	= imagecreatetruecolor( $main_img_obj_w, $main_img_obj_h );
	
		# walk through main image
		for( $y = 0; $y < $main_img_obj_h; $y++ ) {
			for( $x = 0; $x < $main_img_obj_w; $x++ ) {
				$return_color	= NULL;
				
				# determine the correct pixel location within our watermark
				$watermark_x	= $x - $main_img_obj_min_x;
				$watermark_y	= $y - $main_img_obj_min_y;
				
				# fetch color information for both of our images
				$main_rgb = imagecolorsforindex( $main_img_obj, imagecolorat( $main_img_obj, $x, $y ) );
				
				# if our watermark has a non-transparent value at this pixel intersection
				# and we're still within the bounds of the watermark image
				if (	$watermark_x >= 0 && $watermark_x < $watermark_img_obj_w &&
							$watermark_y >= 0 && $watermark_y < $watermark_img_obj_h ) {
					$watermark_rbg = imagecolorsforindex( $watermark_img_obj, imagecolorat( $watermark_img_obj, $watermark_x, $watermark_y ) );
					
					# using image alpha, and user specified alpha, calculate average
					$watermark_alpha	= round( ( ( 127 - $watermark_rbg['alpha'] ) / 127 ), 2 );
					$watermark_alpha	= $watermark_alpha * $alpha_level;
				
					# calculate the color 'average' between the two - taking into account the specified alpha level
					$avg_red		= $this->_get_ave_color( $main_rgb['red'],		$watermark_rbg['red'],		$watermark_alpha );
					$avg_green	= $this->_get_ave_color( $main_rgb['green'],	$watermark_rbg['green'],	$watermark_alpha );
					$avg_blue		= $this->_get_ave_color( $main_rgb['blue'],	$watermark_rbg['blue'],		$watermark_alpha );
					
					# calculate a color index value using the average RGB values we've determined
					$return_color	= $this->_get_image_color( $return_img, $avg_red, $avg_green, $avg_blue );
					
				# if we're not dealing with an average color here, then let's just copy over the main color
				} else {
					$return_color	= imagecolorat( $main_img_obj, $x, $y );
					
				} # END if watermark
		
				# draw the appropriate color onto the return image
				imagesetpixel( $return_img, $x, $y, $return_color );
		
			} # END for each X pixel
		} # END for each Y pixel
			
		# return the resulting, watermarked image for display
		return $return_img;
	
	} # END create_watermark()
	
	# average two colors given an alpha
	function _get_ave_color( $color_a, $color_b, $alpha_level ) {
		return round( ( ( $color_a * ( 1 - $alpha_level ) ) + ( $color_b	* $alpha_level ) ) );
	} # END _get_ave_color()
		
	# return closest pallette-color match for RGB values
	function _get_image_color($im, $r, $g, $b) {
		$c=imagecolorexact($im, $r, $g, $b);
		if ($c!=-1) return $c;
		$c=imagecolorallocate($im, $r, $g, $b);
		if ($c!=-1) return $c;
		return imagecolorclosest($im, $r, $g, $b);
	} # EBD _get_image_color()

} # END watermark API
?>

Tạo file test.php trong thư mục watermark với nội dung sau

<?php 
include('watermark.php'); 
$watermark = new watermark(); 
$main_img_obj = imagecreatefromjpeg('main.jpg'); 
$watermark_img_obj = imagecreatefrompng('watermark.png'); 
$return_img_obj = $watermark->create_watermark( $main_img_obj, $watermark_img_obj, 66 ); 
imagejpeg( $return_img_obj, 'result.jpg', 50 ); 
?>

Chạy file test.php, ảnh kết quả sẽ được lưu thành file result.jpg.