It's simple :
Let's imagine that you are trying to upload a file within js framework, ajax request or mobile application (Client side)
Here how to do it using PHP
<?php
$base64String = "kfezyufgzefhzefjizjfzfzefzefhuze"; // I put a static base64 string, you can implement you special code to retrieve the data received via the request.
$filePath = "/MyProject/public/uploads/img/test.png";
file_put_contents($filePath, base64_decode($base64String));
?>