void ob_start(void);
This function will turn output buffering. While output buffering is active there will be no rel output from the script, the output is appended to an internal buffer instead.
The contents of this internal buffer may be copied into a string variable using ob_get_contents(). Real output happens when ob_end_flush() is called and ob_end_clean() will just silently discard the buffer contents.
See also ob_get_contents(), ob_end_flush(), ob_end_clean() and ob_implicit_flush()