check output buffer

Hello! Thanks for being interested in my message~ ^^ I would like to check whether or not the output has been started~ Since we're working in co-operation~ i have to check it so that i can determine if i am able to set cookie or not~ and one more! Is there any way to clear the buffer that is supposed to be sent to user but not yet~? i thought flush might've been, but it clears the buffer and sends it to users... I like to clear the buffer, and start new output in case of error situation~ in summary, 1. I would like to know whether or not the output started! 2. I would like to know if I can clear the output buffer and start new one! Thank you~~~