I get an error in the production environment of php7.
It is an inquiry function in zeromail.php.

Since I don't touch php in the source of the person who made it before (* wordpress, scss, css, html are fine), I have no idea how to solve the error.

The browser says that there is an error in header ('Location:'. SUCCESSPAGE) ;, but I don't know how to fix it.

Please kindly advise.

Warning: Cannot modify header information --headers already sent by (output started at directory zeromail.php: 676) in directory

// Automatic reply body shaping
    if (strpos (PHPVER, '5') === false) {
        $replymessage = unhtmlentities ($replymessage);
    } else {
        $replymessage = html_entity_decode ($replymessage, ENT_QUOTES,'UTF-8');
    }
        $replymessage = str_replace ("\ r", "", str_replace ("<br />", "", $replymessage));
        @mb_send_mail ($formitem ['email'], REPSUBJECT, $replymessage, $replyheader);
    }
    if (is_admin ()) zeromail_data_put_csv ($csv);// Save CSV
    session_destroy ();
    if (NOSCRIPT === false) {
        print $endMassage;
    } else {
        header ('Location:'. SUCCESSPAGE);
    }