I was creating an associate array the other day. Riveting, I know.

I needed to sort this array by a piece of data, and I was lazy (like all healthy programmers), so I decided to prepend the data to a unique string and make that the key. Nailed, it sorts, right?

No. PHP doesn't realize that 1 is less than 12 when you're sorting strings. The solution I chose for sorting strings of numbers was to add leading zeros. The functions below can be used to do so.

PHP

function leading_zero($number, $num_digits) {		
	return str_repeat("0",($num_digits + -1 - floor(log10($number)))) . $number; 
}

JavaScript

function leading_zero(number, num_digits) {
	var zero = '0';
	var nat_log = Math.floor(Math.log(number)/Math.log(10));
	return zero.rep(num_digits - nat_log - 1) + '' + number;
}

function str_rep(n) {
	var s = "", t = this.toString()
	while (--n >= 0) s += t
	return s
}
String.prototype.rep = str_rep

note: the javascript string repeating code courtesy of Web Refrence.

What's going on here

First I'm finding the base 10 log of $number. This can be done easily with PHP/JavaScript

log ~10~ of $number

$number	log 10	PHP	Javascript
1	0	log10(1)	Math.log(1)/Math.log(10)
10	1	log10(10)	Math.log(10)/Math.log(10)
16	~1.204	log10(16)	Math.log(16)/Math.log(10)
41	~1.613	log10(41)	Math.log(41)/Math.log(10)
125	~2.097	log10(125)	Math.log(125)/Math.log(10)
1337	~3.126	log10(1337)	Math.log(1337)/Math.log(10)

See the pattern yet? The log ~10~ of a number is about 1 less than the number of digits in the number.

Now I run the floor() function on the result of our log ~10~ to truncate the decimal and give us an easy-to-work-with integer.

floor() of log ~10~

$number	log 10	floor()
1	0	0
10	1	1
16	~1.204	1
41	~1.613	1
125	~2.097	2
1337	~3.126	3

Nice. Now we can subtract our floor()'ed number from $num_digits, and then subtract an additional 1, and we have the resulting number of leading zeros to reach $num_digits (example: $num_digits = 4).

The art of subtraction

$number	floor(log 10)	$num_digits	$num_digits – floor(log 10) – 1
1	0	4	3
10	1	4	2
16	1	4	2
41	1	4	2
125	2	4	1
1337	3	4	0

Having determined how many leading zeros we need, use a string repeating function (PHP: str_repeat(), Javascript: um, see the example above), and concatenate the string of zeros with your original number.

Update

My Twitter buddy ceejayoz has a PHP native solution for this:

@steyblind Psssst. $padded = str_pad($string, $length, '0', STR_PAD_LEFT);