inserting string in to variable and echoing it

The thing i trying to do is to insert string in to variable and then echo it.
Lets say i want to echo $string123, but i only know what its starts from $string, and then user specifies how its ends. So i need somehow add after $string what user specified. Any ideas how to do what?
And if not, is there any other ways to do same thing?
Thanks.

 

use an array it is the easiest way $stirng[user_name]=value;

 

do you mean like this?

$my_code = 'test';
$user_code = '123';

$string = $my_code . $user_code;

echo $string;

Code (markup):

or..

$user_code = '123';

$string = 'test';
$string .= $user_code;

echo $string;

Code (markup):