AJAX get call always returning empty string - php

Question

I am trying to make a simple AJAX GET call to my php backend, it hit and runs the method defined however no matter what the response data in the success function is always an empty string with a 200 response.
My ajax request is:
$("#coverage-table").on("click", "td", function() {
$(this).attr('id');
//Create Ajax call
//Get bill data/notes
//Present modal
$.ajax({
url: 'http://tms-v2.test/tms/getBillNotes',
type: 'GET',
data: {
bills: $(this).attr('id')
},
success: function(response) {
console.log(response);
debugger;
modal.style.display = "block";
}
});
});
My php method is:
public function getBillNotes() {
$bills = array_filter(explode("," ,$_GET['bills']));
$billingGateway = new BillingGateway;
$data = $billingGateway->getBillNotes($bills);
//Convert mysql object to array
while($row = mysqli_fetch_array($data)){
$items[] = $row;
}
foreach ($items as $key => $bill) {
$return[$bill['bill_id']] = [
'invoice_number' => $bill['invoice_number'],
'supplier' => $bill['supplier_name'],
'creation_date' => $bill['creation_date'],
'uploaded_by' => $bill['first_name'].' '.$bill['last_name'],
'is_credit_note' => !!$bill['type'],
'validation_status' => !!$bill['is_validating'],
'paid_date' => $bill['paid_date'],
'critical_notes' => $bill['note']
];
}
return 'TEST';
}
However this is always returning "", is this something to do with my request headers?