//获取源图gd图像标识符  $srcImg = imagecreatefrompng('./src.png');  $srcWidth = imagesx($srcImg);  $srcHeight = imagesy($srcImg);    //创建新图  $newWidth = round($srcWidth / 2);  $newHeight = round($srcHeight / 2);  $newImg = imagecreatetruecolor($newWidth, $newHeight);  //分配颜色 + alpha，将颜色填充到新图上  $alpha = imagecolorallocatealpha($newImg, 0, 0, 0, 127);  imagefill($newImg, 0, 0, $alpha);    //将源图拷贝到新图上，并设置在保存 PNG 图像时保存完整的 alpha 通道信息  imagecopyresampled($newImg, $srcImg, 0, 0, 0, 0, $newWidth, $newHeight, $srcWidth, $srcHeight);  imagesavealpha($newImg, true);  imagepng($newImg, './dst.png');