我的代码为使PNG与alpha

$x = 1;
$y = 1;
$gd = imagecreatetruecolor($x,$y); 
imagesetpixel($gd,imagecolorallocatealpha($gd,200,1));
imageAlphaBlending($gd,false);
imageSaveAlpha($gd,true);
imagepng($gd,"test.png");
imagedestroy($gd);

我的代码读PNG alpha

$im = imagecreatefrompng("test.png");
$rgb = imagecolorat($im,0);
$colors = imagecolorsforindex($im,$rgb);
$red = (int) $colors["red"]; 
$blue = (int) $colors["blue"];
$green = (int) $colors["green"]; 
$alpha = (int) $colors["alpha"]; // return only 0

我不知道为什么只返回0而不是1.