Reply To: Undefined index: page

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That’s a possible cause of your problem. If this is indeed what is causing your problem, then the solution is to check if $_GET['page'] exists before you try to use it:

<?php
// add this "isset" check:
if( isset( $_GET['page'] ) ){

    if( $_GET['page']=='home' ){ 
        include "modul/home.php";
    }
    elseif( $_GET['page']=='detail' ){ 
        include "modul/detail.php";
    }

}