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：

• outside of the <?php ?> tags
• printed by language constructs and functions whose explicit purpose is to output user provided variables or strings such as echo, print, printf(), var_dump(), var_export(), vprintf()
• printed by functions whose purpose is to collect and output data/information on the running script or PHP such as debug_print_backtrace(), phpcredits(), phpinfo(), ReflectionExtension::info()
• printed by PHP on an uncaught exception or an unhandled error (subject to the settings of display_errors and error_reporting)
• anything written to php://output

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().