isset causing getjson to return undefined - php

Question

I'm having a problem with my code. I want to validate my form by creating an array to contain a variable when the form is valid. But to do this I need to use the isset method to know that the information has been posted. Here's a simple example
http://richbaird.net/clregister
<?PHP
if(isset($_POST['username'])) {
$helloworld = array ("hello"=>"world","name"=>"bob");
print json_encode($helloworld);
};
if(!isset($_POST['username'])) {
echo json_encode(array('error' => true, 'message' => 'No username specified'));
?>
Simple enough if username has been posted create array helloworld.
I'm using the following method to get the json
<script>
//document ready
$(document).ready(function(){
var php = "helloworld.php";
//submit form
$("#loginform").ajaxForm
(
//on successful submission
function() {
//getjson
$.getJSON("helloworld.php",function(data) {
alert(data.message)
}) //close get json
.error(function(error) { alert(error.responsetext); })
.complete(function() { alert("complete"); });
} // close success
) // close submit
});
//end document ready
</script>
I'm using the jquery forms plugin to submit the form.
and my form looks like this
<form id="loginform" name="loginform" method="post" action="helloworld.php">
<label for="username">username</label>
<input type="text" name="username" id="username" />
<br />
<label for="password">password</label>
<input name="password" type="password" />
<br />
<input name="submit" type="submit" value="Login" id="subtn" />
</form>
the network console shows the method POST returns {hello:world name:bob} but the GET returns the no username specified which is what I get in my alert. It looks like jquery is trying to get the code before it has a chance to process entirely, how can I prevent this?

Answer 1

You're missing the quotes. Should be:
if(isset($_POST['username']))
You should check your console to see whether username is actually getting posted, as if it's not, you're not returning any data. You could instead consider returning an error if(!isset($_POST['username'])), perhaps something like:
echo json_encode(array('error' => true, 'message' => 'No username specified'));
EDIT
Also, remember it's $_POST, not $_post
Second Edit
Your code will be far more intuitive and readable written like this:
$return = array();
if(isset($_POST['username'])) {
$return = array("hello"=>"world","name"=>"bob");
} else {
$return = array('error' => true, 'message' => 'No username specified');
}
echo json_encode($return);

Answer 2

After a few hours of thinking and a lot of help from juco I realized, I'm making 2 seperate calls in this function. First I'm posting the data which works, then on a successful post I'm trying to make a seperate call, a GET request, that request contains the callback which is supposed to alert me of the result, however because it makes a 2nd call and it sends a GET request the variable POST is never set and therefore there is nothing to get back. I revised my code to only use the post method like so.
<script>
//document ready
$(document).ready(function(){
// bind form using ajaxForm
$('#loginform').ajaxForm({
// dataType identifies the expected content type of the server response
dataType: 'json',
// success identifies the function to invoke when the server response
// has been received
success: processJson
}
);
function processJson(data) {
alert(data.hello);
}
});
//end document ready