Pass Javascript variable to PHP via ajax

Since you're not using JSON as the data type no your AJAX call, I would assume that you can't access the value because the PHP you gave will only ever be true or false. isset is a function to check if something exists and has a value, not to get access to the value.

Change your PHP to be:

$uid = (isset($_POST['userID'])) ? $_POST['userID'] : 0;

The above line will check to see if the post variable exists. If it does exist it will set $uid to equal the posted value. If it does not exist then it will set $uid equal to 0.

Later in your code you can check the value of $uid and react accordingly

if($uid==0) {
    echo 'User ID not found';
}

This will make your code more readable and also follow what I consider to be best practices for handling data in PHP.

Pass the data like this to the ajax call (http://api.jquery.com/jQuery.ajax/):

data: { userID : userID }

And in your PHP do this:

if(isset($_POST['userID']))
{
    $uid = $_POST['userID'];

    // Do whatever you want with the $uid
}

isset() function's purpose is to check wheter the given variable exists, not to get its value.