PHP 7.4升级至PHP 8后出现HTTP ERROR 500问题求助

网站从PHP 7.4升级至PHP 8版本后出现HTTP ERROR 500报错，切换回PHP 7.4及更低版本时可正常运行。

• PHP 8下的报错截图：
• PHP 7.4下的正常截图：

以下是我的index.php代码：

<?php
define('INDEX',TRUE);
$bem_cfg = Array();

$bem_cfg['lang'] = Array();

/* current dir aka current app */
$bem_cfg['app_name'] = 'public';

$bem_cfg['app_dir'] = dirname(__FILE__);

require_once $bem_cfg['app_dir'] . '/config.php';

require_once $bem_cfg['app_dir'] . '/app.init.php';

$bem_cfg['tpl_dir'] = './tpl/' . $bem_cfg['cfg_current_template'] ;

require_once $bem_cfg['app_dir'] . '/inc/init.php';

require_once $bem_cfg['lang_dir'] .  '/init.lang.php';

/* init admin menus & modules */
require_once './public.init.php';

$RainTpl->assign('lang',$lang);
$alert = Array();
if(isset($_SESSION['bem_alert'])){
    $alert = $_SESSION['bem_alert'];
    unset($_SESSION['bem_alert']);
}
$RainTpl->assign('alert',$alert);
$RainTpl->assign('locals',$bem_cfg['locals']);

$RainTpl->assign('noMenu',( intval($BemHelpers->getG('nomenu',0)) == 1 ? 1 :0) );



$modPath =  $bem_cfg['mod_dir'] .  '/' . $bem_cfg['app_name'] .
            '/' . $modules[$bem_cfg['app_name']]['main']['class_file_name'];


if( isset( $modules[$bem_cfg['app_name']][($page = $BemHelpers->getG('page','main' ))] ) ) {
    $modPath = $bem_cfg['mod_dir'] . '/' . $bem_cfg['app_name'] . '/' .
     $modules[$bem_cfg['app_name']][$page]['class_file_name'];
}


if( is_file( $modPath ) && file_exists( $modPath ) ) {
    require_once $modPath;
}else{
    $BemHelpers->headers('404');
    die( '<center><span style="font-size:15em">404</span></center>' );
}

if( isset( $CURRENT_MOD_CLASS ) && class_exists( $CURRENT_MOD_CLASS ) ) {
    $currentClass = new $CURRENT_MOD_CLASS( $bem_cfg , $BemHelpers , $BemDB , $RainTpl );
    $currentClass->run();
}else{
    $BemHelpers->headers('404');
    die( '<center><span style="font-size:15em">404</span></center>' );
}

恳请协助排查并解决该问题。

内容的提问来源于stack exchange，提问作者Mohamed Garaihy