jquery.ajax with php

I have just started working on php. It's a very good lang as I'm feeling but some point I get stuck as I'm new to this.

My javascript code

var pv = $("#txtStart").val();
var av = $("#txtStartNextLevel").val();
var au = $("#fileStartPlay").val();
alert(pv+" "+av+" "+au);
var myau = au.split('\\');
$.ajax({
    type:"POST",
    url:php_url,
    data:"{startPoint:"+pv+"nextLevelPoint:"+av+"audioFile:"+myau[myau.length-1]+"}",
    contentType:"application/json",
    dataType:"json",
    success:function(){
        alert("done");
    },
    error:function(){
        alert(response);
    }
});

My PHP code.

<?php
    if(file_exists("Text.txt"))
    {
        $fileName = "Text.txt";
        $fh = fopen($fileName,"a")

        $Starts = $_POST["startPoint"];
        $NextLevel = $_POST["nextLevelPoint"];
        $AudioFileName = $_POST["audioFile"];
            $code .=$Starts."*".$NextLevel."_1*".$AudioFileName."\"";
            fwrite($fh,$code);
        fclose($fh);   
    }
?>

When I run this it executes but doesn't write the values in the variable

$Starts,$NextLevel,$AudioFileName**.

And further if I write the same ajax procedure in

$.post(php_url,{startPoint:pv,nextLevelPoint:av,audioFile:myau[myau.length-1]},function(data){});

this works fine and write the content in the file.

Also As I'm using post method it should not display the values in Address bar what I'm passing to write. But it's showing those values in both the method.

localhost://myphp.php?txtStart=Start&fileStartPlay=aceduos.jpg&txtStartNextLevel=adfd

Please guide me where I'm lacking...