How do i make a if statement that deny a username longer then 10 characters

Im creating a register and login system but i need to know how i make a if statement that first counts the characters and see if it is longer then XX values.

I've tried a lot of stuff but i could'nt find it out. I've seen some youtube tutorials but none of them really answers this question..

The one that i though would work was this:

if( $username > 30 === true ) { ... }

But it didn't work, so now i have no idea how i should do this, i need help...