How to pass variables back using jquery .post from a php file?

In the PHP just echo a variable, ie echo $testVariable; and the value of $testVariable is 10, do this in your jquery

$.ajax
                        ({
                        type: "POST",
                        url: "order.php",
                        dataType: "json",
                        data: postData,
                        cache: false,
                        success: function(testVariable)
                            {
                                alert("Order Saved");
                                $('#assigned_id').html(testVariable);
                            }
                        });

assigned_id now has the value of 10 in it!

What you need to do is find a way for the php script 1 to send post data through ajax.. You can do this by doing:

<?php
    $variable = 0;
?>
<script type="text/javascript">
$(document).ready(function(){
    $('#input').ready(function(){
        $.post("phpscript2.php", {db_code: ""+ $(this).val() +""}, function(data){
                           if(parseInt(data) == 1){
                               $('.message').html('Database updated in good terms');
                           } else {
                               $('.message').html('Sorry the attempt was a fail.');
                           }
         });
    });
});           
</script>

<input type="hidden" value="<?php echo $variable; ?>
<div class="message">Please wait while we process</div>

Then on PHP script 2 you would have:

<?php
    $variable = $_POST['db_code'];

    // Do some code with the posted data that is valued to 0...
    $new_variable = 1;


    echo $new_variable;
?>

This would submit the hidden input field when it loads (php loads before jquery so hidden field will have the value of hte php variable). It will send the value of the hidden field POST using $.post the way you wanted it to, and will send php script 2 the value of the variable in php script 1. Which you will then process the posted data in which ever way you want to do it. Then echo out a new variables value or even can just echo 1, then the echo will return to ajax and there we will parse the data given back, which is 1 but it was echoed so it became a string, and we need to parse it into a integer, check it with a if else statement saying if 1, do this, else do this, setting the message classed div with the success or fail messages. The great thing is that the hidden input can not be edited as it sends the value post, right when the browser loads giving the client no time to edit the value.

Thats the theory anyways. This isnt tested, and im still fairly new at the ajax scene.

Hope i helped!

As you said, the way to show the outcome of a jQuery.post(...) is to use the build in callback. This way you can always "pass back ... php variables". Remember that the PHP can never directly return the variables, it has to be rendered into a finished page or if you choose another response type a JSON structure, XML document or whatever format suits your needs.

If you would like to return a javascript block then I think you could do something like this:

$.ajax({
   type: "GET",
   url: "myphp1.php",
   dataType: "script"
 });

But you could also use dataType "html":

jQuery docs:

If html is specified, any embedded JavaScript inside the retrieved data is executed before the HTML is returned as a string. Similarly, script will execute the JavaScript that is pulled back from the server, then return the script itself as textual data.