php如何实现图片格式转换

/**

 * 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');