Using jQuery $.ajax() on my site won't work
In Drupal, JavaScript files are added via the drupal_add_js() function or in the
theme’s .info file. 
A simple example of using jQuery in drupal is

    &lt;?php
    drupal_add_js('jQuery(document).ready(function () {
   jQuery.ajax({
        url: '/code/test-ajax.php',
        data: {'a':'hello','b':'world'},
        type: 'POST',
        success: function(data){
            alert(data); //Should return "hello, world"
        },
        error: function(jqXHR, textStatus, errorThrown){
            alert(textStatus + errorThrown); //Better diagnostics
        }
    });
    });', 'inline');
    ?&gt;


you have to use 'jQuery' instead of $.