how to pass variables from php to jquery

Question

Why i can't pass the variable co2 to jquery function? nothing shows up where's wrong ? i want to use co2 variable in my jquery function.

  <?php
  ... 
$co2 = (($miles / 41.986) * 20.88 * 2.7) / 2204.6; 
echo json_encode(array('co2' => $co2));
   ?>

jquery:

   $("#aircalc").click(function(){
        var dept = $("#dept").val();
        var dest = $("#dest").val();
        $.post('airtravel.php',{dept: dept, dest: dest}, function(data){
        var a = data.co2;
        $("#airanswer").html(a);

                });

        });

Answer 1

Try this code.

 $("#aircalc").click(function(){
        var dept = $("#dept").val();
        var dest = $("#dest").val();
        values: [ <?php echo $co2; ?>],
        $.post('airtravel.php',{dept: dept, dest: dest}, function(data){
                var a = data.values[0];


                $("#airanswer").html(a);

            });


    });

Answer 2

you can not access co2 because you are calling regular post ajax and return json object you need to call ajax request with json type something like this

$.ajax({
  type: "POST",
  dataType: "json"
  url: 'airtravel.php',
  data: {dept: dept, dest: dest}
})
  .done(function( data ) {
        var a = data.co2;
        $("#airanswer").html(a);
  });