Create a true grayscale PNG in PHP

Question

I need to create a grayscale image in PHP. I am not talking about an indexed image with grayscale values in its palette, but about a TRUE grayscale image. The difference is in the 26th byte of the PNG (color type):

0 - greyscale  <-- THIS IS WHAT I NEED
2 - RGB
3 - RGB with palette
4 - greyscale + alpha
6 - RGB + alpha

(See How to check a PNG for grayscale/alpha color type? for details)

I tried imagefilter($im, IMG_FILTER_GRAYSCALE); as well as imagetruecolortopalette($im, false, 255); but all I get are either RGB grayscale images (color type 2) or RGB palette images with a grayscale palette (color type 3). I also tried to initialize the image with imagecreate() instead of imagecreatetruecolor() but again this only leads to a palette image.

Is there any way to create a color type 0 grayscale PNG with PHP's GD functions (or any other functions in PHP)?

Here are some samples of different grayscale images to show what I mean. They all look the same, but if you open them in PhotoShop and look at the Image -> Mode setting, you see the difference. Also a hex editor will reveal the difference in the 26th byte:

RGB, color type 2, 3149 bytes
RGB palette, color type 3, 3971 bytes
True grayscale image, color type 0, 1105 bytes <-- THIS IS WHAT I NEED

UPDATE 01:

Here is the basic code that I use to create the PNGs. Commented lines are alternatives that I have tried:

//$im = imagecreate($image_size, $image_size);
$im = imagecreatetruecolor($image_size, $image_size);

//imagefilter($im, IMG_FILTER_GRAYSCALE);
//imagetruecolortopalette($im, false, 255);

imagepng($im, $imgPathName);
imagedestroy($im);

Answer 1

GD library does not support converting to a "true" grayscale. It only supports RGB and TrueColor*.

Is there any way to create a color type 0 grayscale PNG with PHP's GD functions (or any other functions in PHP)?

ImageMagick is what you are looking for.

$im = new Imagick();
$im->readImage('file.png');
$im->setImageType(Imagick::IMGTYPE_GRAYSCALE);
$im->writeImage('file.gray.png');