php - Jquery ajax POST response is null -

i have js script ajax request , posts data php script, script echo depending if works or not.

here js

$(document).ready(function(){         var post_data = [];         $('.trade_window').load('signals.php?action=init');          setinterval(function(){             post_data = [   {market_number:1, name:$('.trade_window .market_name_1').text().trim()},                     {market_number:2, name:$('.trade_window .market_name_2').text().trim()}];              $.ajax({                         url: 'signals.php',                         type: 'post',                         contenttype: 'application/json; charset=utf-8',                         data:{markets:post_data},                         datatype: "json",                         success: function(response){                             console.log("response "  + response);                         },                         failure: function(result){                             console.log("failed");                             console.log(result);                         }             });         }, 6000);      }); 

here php:

if(isset($_post["json"]))     {         $json = json_decode($_post["json"]);         if(!empty($json))         {                 echo "it worked!!!!";         }             else                 echo "not posted";     } 

so basically, thought response in `success: function(response)' method populated either "it worked!!!" or "not posted" depending on if statement in php. seem work because js script manages go success statement prints console:

response null

i need able return server in order update screen.

any ideas i'm doing wrong?

try:

if(isset($_post["markets"])) {     $json = json_decode($_post["markets"]);     if(!empty($json))     {             echo "it worked!!!!";     }         else             echo "not posted"; }