Yeah, what a nightmare figuring that out, could magickwand be any more cryptic..
anyhow, here you go
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<?php
///////////////
// php imagemagick test harness
//
// create a watermarked image
///////////////
//xoffset of the watermark
$xoffset =10;
//yoffset of the watermark
$yoffset = 10;
//the original image to be watermarked
$originalimage ="original.jpg";
//the watermark to apply
$watermarkimage = "watermark.png";
//top end opacity number (totally transparent)
$opacity0 = @MagickGetQuantumRange();
//bootom opacity (totally visible)
$opacity100 = 0;
//desired opacity percentage
// THIS IS THE ONE TO SET
$opacitypercent = 70;
//gather the actual opacity number
$opacity = $opacity0 - ($opacity0 * $opacitypercent/100 ) ;
//validate the opacity number
if ($opacity > $opacity0){
$opacity = $opacity0;
}elseif ($opacity <0){
$opacity = 0;
}
//initialize the wands
$sourceWand = NewMagickWand();
$compositeWand = NewMagickWand();
//read in the images
@MagickReadImage($compositeWand, $watermarkimage);
@MagickReadImage($sourceWand, $originalimage);
//setting the image index
MagickSetImageIndex($compositeWand, 0);
MagickSetImageType($compositeWand, MW_TrueColorMatteType);
//seting the opacity level
MagickEvaluateImage($compositeWand, MW_SubtractEvaluateOperator, $opacity, MW_OpacityChannel) ;
//combining the images
@MagickCompositeImage($sourceWand, $compositeWand, MW_ScreenCompositeOp, 0, 0);
//print out the image
header("Content-Type: image/jpeg");
MagickEchoImageBlob($sourceWand);
?>