How to Communicate between PHP and JavaScript -

i wrote javascript function convert geojson data wkt format. works when input value in javascript code directly. don't know how input php , send back.

here php code:

<?php   $geojson=file_get_contents("clipfeature.geojson");   $wkt = $_post['wkt'];   echo ($wkt); ?> 

so gets geojson data file , want receive converted wkt code javascript function.

please me finish javascript code:

   function converttowkt (){      $.ajax({         type: "get",         url: "readjson.php",         contenttype: "application/json"     }).done(function (data) {      var jsonobject = how give value php variable;      var coordinate = jsonobject.features[0].geometry.coordinates;     var type= jsonobject.features[0].geometry.type;       var coordinate1 = "";     var coordinate2 = "";     (var i=0; i< coordinate[0].length; i++) {      coordinate1= coordinate[0][i][0]+" "+coordinate[0][i][1];      coordinate2=coordinate1+","+coordinate2;     }      var wkt= "\""+ type + "((" + coordinate2;         wkt=wkt.substring(0,wkt.length-1);         wkt=wkt+"))\""       sendback ( );  });       function sendback(){$.post("readjson.php",     {'wkt':how send value of var wkt php 'wkt'     });     } 

basically:

var jsvar= "<? echo $myvariable_value_goes_in_here; ?>"; 

or, mentioned above:

var jsonobject =  <? echo json_encode($wkt); ?>;                     // need json, otherwise syntax error in js!