Ajax发布NAN值

I'm posting a variable via ajax however when I use it in PHP, I get NAN why is that? it's supposed to be a number

//in js

var variableToSend = 1;
$.post('ajax.php', {variable: variableToSend});

//in php

<?php echo $_POST['variable']?>;

This code won't work,

Because AJAX is something that works in the background, So there is no way the "echo " in PHP will display the data, Because the data returns back to the success:function and then that can be consoled or printed on the HTML

Here is an example code of how it works:

var variableToSend = 1;
$.ajax({
  url:"ajax.php",
  method:"POST",
  data: {
    variable:variableToSend
  },
  success:function(data){
    console.log(data);
  }
})

Hope you understood how AJAX works