How to add leading zeros to a number

Short answer

str_pad() pads a string on the left to the needed length:

<?php
 
echo str_pad('7', 3, '0', STR_PAD_LEFT);

Output:

007

With sprintf

The %05d format means "an integer five characters wide, zero-padded":

<?php
 
echo sprintf('%05d', 42);

Output:

00042

A time

Hours and minutes with zeros:

<?php
 
echo sprintf('%02d:%02d', 9, 5);

Output:

09:05