PHP GD: merge a blurry rectangle - php

Question

I need to merge a blurry rectangle on another image (a white rectangle). I tried to imagesavealpha() but unfortunately the background of the rectangle remains black, and I want it with a gradient from red to white.
Here is my code:
<?php
$width = 200;
$height = 180;
$bw = $bh = 30;
$img1 = imagecreatetruecolor($width, $height);
$img2 = imagecreatetruecolor($width, $height);
$white = imagecolorallocate($img1, 255, 255, 255);
$red = imagecolorallocate($img2, 255, 0, 0);
imagefilledrectangle($img1, 0, 0, 100, 100, $white);
imagefilledrectangle($img2, 5, 5, 25, 25, $red);
imagefilter($img2, IMG_FILTER_GAUSSIAN_BLUR);
imagefilter($img2, IMG_FILTER_GAUSSIAN_BLUR);
imagefilter($img2, IMG_FILTER_GAUSSIAN_BLUR);
imagefilter($img2, IMG_FILTER_GAUSSIAN_BLUR);
imagefilter($img2, IMG_FILTER_GAUSSIAN_BLUR);
imagesavealpha($img2, true);
imagecopymerge($img1, $img2, 20, 20, 0, 0, $bw, $bh, 100);
header('Content-Type: image/png');
imagepng($img1);
imagedestroy($img1);
The restulting image is:

Answer 1

If you just want a blurred red rectangle on a white background your code can be simplified to this:
<?php
$width = 200;
$height = 180;
$img = imagecreatetruecolor($width, $height);
// fill with opaque white.
imagefill($img, 0, 0, 0x00ffffff);
// draw rectangle in opaque red.
imagefilledrectangle($img, 5, 5, 25, 25, 0x00ff00000);
for ($i = 0; $i < 5; $i++) {
imagefilter($img, IMG_FILTER_GAUSSIAN_BLUR);
}
header('Content-Type: image/png');
imagepng($img);
imagedestroy($img);
Result (of course the white background blends with the page background...):
If you want to be able to blend the red rectangle with any background colour (full alpha blending) then you might be out of luck. As far as I can tell IMG_FILTER_GAUSSIAN_BLUR doesn't support alpha values (I'm using PHP 7.0.3).