ajax error FROM php require_once <--- This has been one of convetional errors. [struggling tens of ideas for 48hrs

ajax error FROM php require_once <--- This has been one of convetional errors. [struggling tens of ideas for 48hrs

If I remove ' require_once' from php script, everything is fine.
If it is included, $.ajax parse error with readystate 4, status 200.

directory structure:

teach > index.php

$.ajax ({
                    type: "POST",
                    url: 'tlog_user/teach_signinemail6.php',     // require_once('mconnectvars1.php');           
                    data: logdata,
                   dataType:"json",                     
   
                   success : function(s) {     
                               alert(s);
                               readystate 4, status 200.
                     
                   },
                   error:function(request, status, error){