/**
 * POST请求-文件
 * @param $url
 * @param $params
 * @return bool|string
 */
private function _ajaxPostImage( $url, $params ) {
   $curl = curl_init();
   curl_setopt( $curl, CURLOPT_URL, $url ); //URL
   curl_setopt( $curl, CURLOPT_HTTPHEADER, ['Content-Type'=>'application/json']);
   curl_setopt( $curl, CURLOPT_RETURNTRANSFER, 1 );
   curl_setopt( $curl, CURLOPT_POST, 1 );
   curl_setopt( $curl, CURLOPT_POSTFIELDS, $params);
   curl_setopt( $curl, CURLOPT_SSL_VERIFYPEER, FALSE );
   curl_setopt( $curl, CURLOPT_SSL_VERIFYHOST, FALSE );
   $response = curl_exec( $curl );
   curl_close( $curl );
   return $response;
}

文档看一下：https://developers.weixin.qq.com/miniprogram/dev/api-backend/customerServiceMessage.uploadTempMedia.html