php图片类型如何转换

/**
 * 图片格式转换
 * @param string $image_path 文件路径或url
 * @param string $to_ext 待转格式，支持png,gif,jpeg,wbmp,webp,xbm
 * @param null|string $save_path 存储路径，null则返回二进制内容，string则返回true|false
 * @return boolean|string $save_path是null则返回二进制内容，是string则返回true|false
 * @throws Exception 
 * @author klinson <klinson@163.com>
 */
function transform_image($image_path, $to_ext = 'png', $save_path = null)
{
 if (! in_array($to_ext, ['png', 'gif', 'jpeg', 'wbmp', 'webp', 'xbm'])) {
 throw new Exception('unsupport transform image to ' . $to_ext);
 }
 switch (exif_imagetype($image_path)) {
 case IMAGETYPE_GIF :
 $img = imagecreatefromgif($image_path);
 break;
 case IMAGETYPE_JPEG :
 case IMAGETYPE_JPEG2000:
 $img = imagecreatefromjpeg($image_path);
 break;
 case IMAGETYPE_PNG:
 $img = imagecreatefrompng($image_path);
 break;
 case IMAGETYPE_BMP:
 case IMAGETYPE_WBMP:
 $img = imagecreatefromwbmp($image_path);
 break;
 case IMAGETYPE_XBM:
 $img = imagecreatefromxbm($image_path);
 break;
 case IMAGETYPE_WEBP: //(从 PHP 7.1.0 开始支持)
 $img = imagecreatefromwebp($image_path);
 break;
 default :
 throw new Exception('Invalid image type');
 }
 $function = 'image'.$to_ext;
 if ($save_path) {
 return $function($img, $save_path);
 } else {
 $tmp = __DIR__.'/'.uniqid().'.'.$to_ext;
 if ($function($img, $tmp)) {
 $content = file_get_contents($tmp);
 unlink($tmp);
 return $content;
 } else {
 unlink($tmp);
 throw new Exception('the file '.$tmp.' can not write');
 }
 }
}

// 转换后保存在test.png
transform_image($url, 'png', './test.png');
transform_image($filepath, 'png', './test.png');
// 转换后二进制结果直接返回
transform_image($url, 'png');
transform_image($filepath, 'png');