How can I check if something was print in the browser? I've tried headers_sent but it's for headers... If nothing was printed i want to download a file:

public function download() {
    $file = null;
    $line = null;

    if(headers_sent($file, $line)) {
        /* generic exception... change that... */
        throw new Exception('Headers was sent before in file ' . $file . ', line #' . $line);
    }

    header('Content-Description: File Transfer');
    header('Content-Type: ' . $this->mime);
    header('Content-Disposition: attachment; filename=' . $this->name);
    header('Content-Transfer-Encoding: binary');
    header('Expires: 0');
    header('Cache-Control: must-revalidate, post-check=0, pre-check=0');
    header('Pragma: public');
    header('Content-Length: ' . $this->size);
    readfile($this->path);

Teşekkür ederim.