使用来自AJAX对象的序列化数据和PHP

Question

I'm sending an entire form, about 8 fields, along with my AJAX data object, one of which is a serialized string:

var fields = $(this).serialize();

var data = { 
    action:'newSubmission',
    nonce: Nonce,
    fields: fields
}; 

Now within PHP I cannot understand how to get each field value from the $_POST object correctly.

$test = $_POST['field'];

Is the full string and sends back to JS a properly formatted JSON object. But how do I break up the serialized string correctly in PHP?