How can I generate a 6 digit unique number?

If you want it to start at 000001 and go to 999999:

$num_str = sprintf("%06d", mt_rand(1, 999999));

Mind you, it's stored as a string.

$six_digit_random_number = random_int(100000, 999999);

As all numbers between 100,000 and 999,999 are six digits, of course.