Send ONE Javascript variable to PHP via AJAX

I've been looking around the forums without a solution to my problem. It's pretty simple, really, and I'd appreciate it if you could explain your answer if you would be so kind.

I'm new to AJAX and Javascript, and I need to send one variable from my javascript code and basically "convert" it into php. Here's what I have so far:

var selected = rowData.ID

jQuery.ajax({
    url: "test.php",
    type: 'POST',
    data: { selected },
    cache: false
});

I use this selected value further down in the code. I use PHP to display the (value of selected).

"vars": [ 
    "(value of selected)" 
],      

However, I can't seem to make my ajax request work and send the variable to my PHP file. Here's what my PHP file looks like:

$row = $_POST["selected"];

Thanks in advance for the help.