username + numero progressivo a 2 cifre

[ken84]

Una cosa così?

$first_name = "pippo";
$last_name = "pluto";
$num = 2;

$username = substr($first_name,0,3)."-".substr($last_name,0,3)."".str_pad($num, 2, "0", STR_PAD_LEFT); 

ciao