Bring your own theme to PHP 8

I have found the error.

In the header.php an undefined constant was tried to be accessed. So instead of the following wrong code:

<?php if ($pagetitle == pagetitle_1){

  $pt1 = "pagetitle_1";

  $main = "main_1";

} else {

  $pt1 = "";

}

?>

I had to add the quotation marks to pagetitle:

<?php if ($pagetitle == "pagetitle_1"){

  $pt1 = "pagetitle_1";

  $main = "main_1";

} else {

  $pt1 = "";

}

?>