imagesavealpha() sets the flag to attempt to save full alpha channel information (as opposed to single-color transparency) when saving PNG images.
You have to unset alphablending
imagealphablending($im, false)), to use it.
Alpha channel is not supported by all browsers, if you have problem with your browser, try to load your script with an alpha channel compliant browser, e.g. latest Mozilla.
An image resource, returned by one of the image creation functions, such as imagecreatetruecolor().