How does php return a value of 000001 instead of 1?

Question

EG: Fixed 6-digit length, 000001, 000002, 000003,......

Increase in turn.

Answer 1

str_pad($number, 6, '0', STR_PAD_LEFT);

Answer 2

sprintf("%06d", $num);

Answer 3

    echo sprintf("%'06d\n", 1);

Answer 4

sprintf("%06d", $num);