I understand, that’s fine. I don’t completely understand why username’s need to be at least 6 characters (@traq, for example, would need to come up with something different) but those decisions are entirely down to you.
The reason your code isn’t working is because you think that
Is a variable and it isn’t. It’s a string.
Will always be 9. Therefore
if(strlen(':username') < 6)
Will always be false.
Is a variable. It starts with a dollar sign and isn’t surrounded by quotation marks.