JSON PHP data returning null

i am trying to fetch a PHP script using AJAX and return the values as JSON. For some reason, my script fails and i am trying to figure out the problem. When i enter a value form the database into the address bar, like

www.someaddress/post.php?kinaseEntry=aValue

i get a json out put like so;

{"kinaseSKU":null,"url":null,"molecularWeight":null,"tracerSKU":null,"antiSKU1":"antiSKU1","antiSKU2":"antiSKU2","bufferSKU":"bufferSKU","tracerConc":null,"assayConc":null}

My php file looks like so;

<?php

//Include connection to databse require_once 'connect.php';

$kinase = mysql_real_escape_string ($_POST["kinaseEntry"]);



mysql_query('SET CHARACTER SET utf8'); 



$findKinase = "SELECT * FROM kbaData where cleanSKU = '" .$kinase. "' ";



if ($result = mysql_query($findKinase)) {



    $row = mysql_fetch_array($result, MYSQL_ASSOC);



    $kinaseSKU = $row['cleanSKU'];

    $url = $row['url'];

    $molecularWeight = $row['molecularWeight'];

    $tracerSKU = $row['tracerSKU'];

    $antiSKU1 = $row['antiSKU1'];

    $antiSKU2 = $row['antiSKU2'];

    $bufferSKU = $row['bufferSKU'];

    $tracerConc = $row['tracerConc'];

    $assayConc = $row['assayConc'];





    /* JSON ROW */

    $json = array ("kinaseSKU" => $kinaseSKU, "url" => $url, "molecularWeight" => $molecularWeight, "tracerSKU" => $tracerSKU, "antiSKU1" => $antiSKU1, "antiSKU2" => $antiSKU2, "bufferSKU" => $bufferSKU, "tracerConc" => $tracerConc, "assayConc" => $assayConc );



} else  {



    /* CATCH ANY ERRORS */

    $json = array('error' => 'Mysql Query Error');

}



/* SEND AS JSON */

header("Content-Type: application/json", true);



/* RETURN JSON */

echo json_encode($json);



/* STOP SCRIPT */

exit;

?>

Am I going about this the wrong way? or have i done this wrong?

EDIT: Here is my jquery/ajax that calls the php script;

$(document).ready(function() {

$('#kinaseEntry').change(function ()    {



    var kinaseEntry = $('#kinaseEntry').val();

    var dataString = 'kinaseEntry' + kinaseEntry;



    $('#waiting').show(500);

    $('#message').hide(0);

    alert(kinaseEntry);



    //Fetch list from database

    $.ajax({

        type : "POST",

        url : "post.php",

        datatype: "json",

        data:   dataString,

        success : function(datas)   {

            alert("datas" + datas);

        },

        error : function(error) {

            alert("Oops, there was an error!");

        }

    });

    return false;

});

});