here the working version of walf's solution
<?php
if (!function_exists('imagepalettetotruecolor')) {
    function imagepalettetotruecolor(&$src) {
        if (imageistruecolor($src)) {
            return true;
        }
        $dst = imagecreatetruecolor(imagesx($src), imagesy($src));
     
        imagealphablending($dst, false);$transparent = imagecolorallocatealpha($dst, 255, 255, 255, 127);imagefilledrectangle($dst, 0, 0, imagesx($src), imagesy($src), $transparent);imagealphablending($dst, true);imagecopy($dst, $src, 0, 0, 0, 0, imagesx($src), imagesy($src));
        imagedestroy($src);
        $src = $dst;
        return true;
    }
}
?>