[php] question about ob_end_flush

Hi all I am comfused about the function ob_end_flush.In the manual:This function will send the contents of the topmost output buffer (if any) and turn this output buffer off. i have made such tests. <?php ob_start(); echo "hello word!"; ob_end_flush(); ?> this works alright.the script output "hello world!". <?php ob_start(); echo "hello word!"; ob_end_flush(); ob_clean(); ?> this script output nothing.i don't know why this heppan.why the ob_clean function can affect the output?ob_end_flush has send the content( in this case it is "hello word!") and turn off the buffer! thanks. Jason