png图片改变大小保存 php

<?php    
	//获取源图gd图像标识符
	$srcImg = imagecreatefrompng('./yuantu.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, './thumb.png');
 
?>