How to open php files through localhost

I am creating a student management program with xampp, PHP, MySQL, and Apache. But the problem is that when I opened my index.html file with localhost: localhost/superschool/index.html, the site address changed into localhost/superschool/index.php. This shows up this error message: HTTP ERROR 500 with console message: (index):7290 crbug/1173575, non-JS module files deprecated.

index.html

<html>
    <head>
        <meta http-equiv="Refresh" content="0;url=index.php">
        <title>Loading ...</title>
    </head>
    <body></body>
</html>

index.php

    error_reporting(E_ERROR | E_WARNING | E_PARSE);
    $currDir = dirname(__FILE__);
    include("{$currDir}/defaultLang.php");
    include("{$currDir}/language.php");
    include("{$currDir}/lib.php");

    $x = new DataList;
    $x->TableTitle = $Translation['homepage'];
    $tablesPerRow = 2;
    $arrTables = getTableList();

    // according to provided GET parameters, either log out, show login form (possibly with a failed login message), or show homepage
    if(isset($_GET['signOut'])){
        logOutUser();
        redirect("index.php?signIn=1");
    }elseif(isset($_GET['loginFailed']) || isset($_GET['signIn'])){
        if(!headers_sent() && isset($_GET['loginFailed'])) header('HTTP/1.0 403 Forbidden');
        include("{$currDir}/login.php");
    }else{
        include("{$currDir}/main.php");
    }

Can someone help me with this issue?