Adding a '/' makes the website crash

This is how it looks without the '/' (localhost/index.php): http://ift.tt/16Lopl0

After adding it at the end of url (localhost/index.php/): http://ift.tt/16Lopl2

Anyone with experience that could just give a short tip about a probable reason of that.

thanks in advance.

Here is the index.php :

       require_once("php/config.php");

       if(substr_count($_SERVER['PHP_SELF'],'/')==1){ // fixing that bug  


          require_once( ROOT_PATH . "php/header.php" );   

          if(isvalid(quizzid()) == true){

                  switch(quizzid()){

                  case 0:

                    require_once( ROOT_PATH . "php/questionarios.php" );

                  break;

                  default:

                     require_once( ROOT_PATH . "php/questionario.php" );  

                  break;

                  } 
         }

             require_once(ROOT_PATH . "php/footer.php");  // footer

          }else{ // if there are more than 1 '/' it would redirect to > index

  header('Location: /index.php');

}