PHP 实现缩略图

/*
$uploaded - 已上传的文件，可以理解为原图片
$uptype - 图片类型
$filename - 生成的缩略图的文件名（可包含路径）
$a_width - 缩略图宽度
$a_height - 缩略图高度
*/
	function creat_thumb($uploaded,$uptype,$filename,$a_width,$a_height)
	{
		$im = '';

		if($uptype == 'image/pjpeg' || $uptype == 'image/jpeg')
		{
			$im = imagecreatefromjpeg($uploaded);
		}
		else if($uptype == 'image/x-png' || $uptype == 'image/png')
		{
			$im = imagecreatefrompng($uploaded); 
		}
		else if($uptype == 'image/gif')
		{
			$im = imagecreatefromgif($uploaded); 
		}

		$width = imagesx($im); 
		$height = imagesy($im);

		//确保原图比要生成的缩略图宽高要大
		//计算宽高比例，哪个值大就按照哪个作为基准
		//如果宽高相等，则忽略
		if($width > $a_width || $height > $a_height)
		{
			if($width >= $height)
			{
				$newwidth = $a_width;
				$newheight = ($height * $a_width) / $width;
				
				
				$nx = 0;
				$ny = 0;
			}
			else
			{
				$newheight = $a_height;
				$newwidth = ($width * $a_height) / $height;
				$nx = 0;
				$ny = 0;
			}
			
			if(function_exists("imagecopyresampled"))
			{ 
				$newim = imagecreatetruecolor($newwidth, $newheight);
				if($uptype == 'image/x-png' || $uptype == 'image/png')
				{
					$alpha = imagecolorallocatealpha($newim, 0, 0, 0, 127);
					imagefill($newim, 0, 0, $alpha);
				}
				imagecopyresampled($newim, $im,  0, 0,$nx,$ny,  $newwidth, $newheight, $width, $height); 
			}
			else
			{ 
				$newim = imagecreate($newwidth, $newheight); 
				imagecopyresized($newim, $im, 0, 0, 0, 0, $newwidth, $newheight, $width, $height); 
			}
			if($uptype == 'image/x-png' || $uptype == 'image/png')
			{
				imagesavealpha($newim, true);
				imagepng($newim,$filename);
			}
			else
			{
				imagejpeg($newim,$filename); 
			}
			imagedestroy($newim);
		}
	}