How to add a percent symbol to a string variable?

In my application, I'm outputting a number. In the real life application, it is a calculation for brevity purposes, I removed that part because it is working correctly. The issue I'm having is I'm trying to do a string concatenation and append a string value of the percent sign to it. Does anyone have any suggestions?

$my_var = "";
$my_var = number_format(($x']),2) + '%';