Passing variables through AJAX

[BigfootNick]

I've got a table:

<table class="table table-hover" id="beta">
	<thead>
		<tr>
			<th>Id</th>
			<th>Model</th>
			<th>AssetTag</th>
		</tr>
	</thead>
	<tbody>
		<tr>
			<td>1</td>
			<td>A</td>
			<td>B</td>
		</tr>
	</tbody>
</table>

that is initialized with this DynaTable code:

<script>
	$(document).ready(function() {
		$('#beta').dynatable({
			dataset: {
				ajax: true,
				ajaxOnLoad: true,
				ajaxUrl: 'load-assets.php',
				records: "records",
				}
		});
	});
</script>

This pulls some JSON data from a SQL query.

<?php
	/*$newCategory = $_POST['newCategory'];*/
	$newCategory = "desktop";
	**SQL Query**
	$json = array();
	while( $row = sqlsrv_fetch_array( $result, SQLSRV_FETCH_NUMERIC ))
	{    
		array_push($json, array('id'=>$row[0],'Model'=>$row[3],'AssetTag'=>$row[4]));
	}
	$json_data = array(
		"records" => $json,
		"queryRecordCount"=>13,
		"totalRecordCount"=>13
	);
	echo json_encode($json_data);
?>

This works (which was a RELIEF after spending a whole day trying to get DataTables to work...) but only if I define my '$newCategory' variable from within this Ajax file.
If I try to pass it from jQuery I get an error:
Notice: Undefined index: newCategory in C:\wamp\www\bootstrap\load-assets.php

I've tried putting newCategory: "desktop" all over the dynatable function, but I can't get it to pass-through.

Also, though it pulls data when I hard-code in that variable value - I'm only requesting three columns: Id, Model, AssetTag, and I get four columns in the resulting table, three of which are filled with 'undefined'
Edit: I figured out why I had 4 columns - now I have three, with two that are filled by 'undefined'

Any thoughts on how I can resolve those?

[BigfootNick]

Never mind about the 'undefined' columns... I changed my query from

while( $row = sqlsrv_fetch_array( $result, SQLSRV_FETCH_NUMERIC ))
	{    
		//array_push($json, array($row[0],$row[3],$row[4]));
		array_push($json, array('id'=>$row[0],'Model'=>$row[3],'AssetTag'=>$row[4]));
		
	}

to this:

while( $row = sqlsrv_fetch_array( $result, SQLSRV_FETCH_NUMERIC ))
	{    
		//array_push($json, array($row[0],$row[3],$row[4]));
		array_push($json, array('id'=>$row[0],'model'=>$row[3],'assetTag'=>$row[4]));
		
	}

I have no idea why that worked, but it did.
Still, how do I pass a variable? I want to input the variable into my SQL query.