Ajax发布到同一页面上的php无法正常工作

I am trying to pass a var from jQuery to PHP.

After researching I found the most common suggestion was to do this with Post with Ajax and store the post var using PHP.

My PHP skills are pretty good but I can't say the same about my JavaScript skills.

This is what I am doing but does not seem to work at all:

  // jquery libray included etc etc
  <script>
   $.ajax({
      type: "POST",
      url: "http://currentpage.com",
      data: "var=value",
      dataType: string
    });
 </script>
 </head>


<body>

<?php
if($_REQUEST["var"] == "value") {
    echo "var passed and stored";
}
?>
<script>
   $.ajax({
      type: "POST",
      url: "index.php",
      data: {var:'value'},
      dataType: 'text',
      success:function(data){
       // Test what is returned from the server
         alert(data);
      }
    });
 </script>

Try

<script>
   $.ajax({
      type: "POST",
      url: "http://currentpage.com",
      data: {variablename:'value'},
      dataType: "text", //Available types xml, json, script, html, jsonp, text
      success:function(response){
       //Returned from server
       alert(response);
      }
    });
 </script>