如何将值从jQuery-AJAX函数发送到PHP文件并在PHP文件中访问这些值？

I've written a jQuery-AJAX function as follows :

$('#request_form').submit(function(e) {
  var form = $(this);
  var stud_id = $('#stud_id').val();
  var reg_date = $('#reg_date').val();
  var formdata = false;


  var fileInput = $("#receipt_image")[0];  

  /*I want to pass values of below variables to the PHP file.*/
  var ImgSizeInBytes = fileInput.files[0].size;
  var filename = $('input[type=file]').val().split('\\').pop();
  var customer_id = $('#customer_id').val();
 /*These values need to send to PHP file and access there */

  if(window.FormData) {
    formdata = new FormData(form[0]);
  }

  var formAction = form.attr('action');

  $.ajax({
    url         : 'student_request.php',
    type        : 'POST',    
    cache       : false,
    data        : formdata ? formdata : form.serialize(),
    contentType : false,
    processData : false,
    success: function(response) {
      var responseObject = $.parseJSON(response);    
      if(responseObject.error_message) { 
        if ($(".alert-dismissible")[0]) {
          $('.alert-dismissible').remove();   
        }  
        var htmlString = "<div class='alert alert-danger alert-dismissible' role='alert'><button type='button' class='close' data-dismiss='alert' aria-hidden='true'>&times;</button>"+responseObject.error_message+"</div>";    
        $(htmlString).insertBefore('div.modal-body #request_form');        
      } else { 
        alert("Student successfully registered...!!!");       
      }
    }
  });
  e.preventDefault();
});

Now I'm able to access the values filled in by user on a form by means of $_POST array in PHP file. But I also want to pass the values I put in comment in my code above to the PHP file.

The values/parameters which I want to send are not part of a form fields. I've manipulated the values of these variables. So they can't come in $_POST array.

My issue is how should I send these values to PHP file and how should I access these values in PHP file?

You should change this: formdata ? formdata : form.serialize()

Store this in a variable and concatenate the values you want to send.

For Example:

var pars = formdata ? formdata : form.serialize();
pars += "&myField1=myValue1&myField2=myValue2"

As @chris said, all you need to do is to concatenate your own hidden variables to post variables. As I see, you are confused about how to use those extra variables in your php file, here's simple example:

var params = formdata ? formdata : form.serialize();
params += "param1=myExtraVar1&param2=myExtraVar2";

So now you have all variables ready to be sent to your php file, modify your data parameter in ajax call like this:

...data: params,

So far, so good. Let's see the other side (PHP)

<?php 
    // get the variables you want to treat.
    $param1 = $_POST['param1']; // now you have access to this variable from ajax call
   // Notice you can display all variables you have in superglobal variable POST 
   // by dumping it using either var_dump($_POST) or print_r($_POST)

Hope this helps understand better the process, and feel free to comment and I'll get back to you

Another thing I captured and I'd like to share with you is that you can use datatype to JSON instead of casting your returned response, so you can put this code anywhere inside your ajax call:

dataType: "json", // if you put this in last line, omit the comma, otherwise leave as it is