headers_sent

 bool headers_sent ([ string &$file [, int &$line ]] ) 

説明

Checks if or where headers have been sent. You can't add any more header lines using the header() function once the header block has already been sent. Using this function you can at least prevent getting HTTP header related error messages. Another option is to use Output Buffering.

戻り値

headers_sent() will return FALSE if no HTTP headers have already been sent or TRUE otherwise.