I have a function that generates a random number, like:

$int = random_number();

Generally, the results are 6 digits, but occasionally, it ends up being less than 6 digits. How do I add zeros to the number if it is less than 6 digits? For example, if $int is 12345, I turn it into 123450.

I can easily do this with something like:

if (strlen($int) == 5) {
   $int = $int.'0';
} elseif(strlen($int) == 4){
   $int = $int.'00';
} //etc

But clearly there must be a better way?

strpadding this is called.

See here: https://secure.php.net/manual/en/function.str-pad.php

Example:

echo str_pad($yourRandomNumber, 6, "0", STR_PAD_LEFT);

Usually sprintf works for this purpose, but adds a leading zeros what is not a OPs question.

$padded = sprintf('%06d', 123);

For trailing zeros with sprintf as brilliantly mentioned by Ivar (in comments section below) the following code will work:

$padded = sprintf('%-06s', 123);

UPD

For adding trailing zeros str_pad can be used.

$padded = str_pad($num, 6, '0', STR_PAD_RIGHT);

Note: STR_PAD_RIGHT is a default argument, so can be omitted.

Or you can use rand(100000,999999); if you're not using random number with extra parameters.

$number = "878"; //the random number
$int = strlen($number);
$multiplier = 6 - $int;
if ($multiplier != 0) {
    $number = $number * pow(10 , $multiplier);
}

Its simple we just multiply with 10*10*10 (the number of missing numbers).