PHP中的output_buffering可以堆砌, 即可以多次调用ob_start()以开启缓冲区,具体参照手册中ob_start()的介绍,相关摘录如下:
Output buffers are stackable, that is, you may call ob_start() while another ob_start() is active. Just make sure that you call ob_end_flush() the appropriate number of times. If multiple output callback functions are active, output is being filtered sequentially through each of them in nesting order.
php.ini中的output_buffering不为Off时, 会自动开启一个缓冲区(等同于调用一次ob_start()), 如果为数值, 即为缓冲区的缺省大小.
ob_get_contents()在未开启缓冲区的情况下调用不报错, 返回false.而ob_end_clean()以及相类似操作会报告Notice级别错误, 告知未开启缓冲区.
