缓冲什么输出?

PHP 的用户级输出缓冲区在启动后缓冲所有输出,直到它们关闭或脚本结束。PHP 用户级输出缓冲区中上下文中的输出是 PHP 将显示或发送回浏览器的所有内容。实际上,输出是非零长度数据,即:

注意: Data that is written directly to stdout or passed to an SAPI function with a similar functionality will not be captured by user-level output buffers. This includes writing data to stdout with fwrite() or sending headers using header() or setcookie().