//post data
$post = $_POST['username'];
//str replace
$print = str_replace([1,2,3,4,5], ['4A','6B','7C','2D','6F'], $post);
//prints result
echo "{\"username\":\"" . $print . "\"}";
str replace works fine when it's just alpha (example: TEST!@#test = 7160767124452671607671
which is just perfect, but as soon as digits come into play things get messy $print
should output {"username":"4A6B7C2D6F"}
if input 12345
the current output is {"username":"2DA6B7C2D6F"}