Solution to Black Transparent Background of php Scale gif and png Images

In the work, I need to zoom some gif pictures and then go to Imagecopymerge, but I found that after using imagecreatetruecolor and imagecopyresampled, I found that the background picture was wrong, and the original transparent background picture turned black. Later, I found that I could make some modifications:

 $img = imagecreatetruecolor(200, 200); 
//2. Coloring  
$color=imagecolorallocate($img,255,255,255); 
//3. Setting Transparency  
imagecolortransparent($img,$color); 
imagefill($img,0,0,$color); 

Then imagecopyresampled and Imagecopymerge will be no problem