IMagick is a native PHP extension to create and modify images using the ImageMagick API. ImageMagick Studio LLC did not write nor does it maintain the IMagick extension, however, IMagick users are welcome to discuss the extension here.
By adding PNG24 your are turning off the alpha channel in your result so the background will not be transparent. The following works for me if you want grayscale alpha result.
You cannot start with an image that has its transparency replaced by some checkerboard pattern. I showed that it works above if you start with a proper transparent image.