Zero-pad digits in string

Question

First of all, your description is misleading. Double is a floating point data type. You presumably want to pad your digits with leading zeros in a string. The following code does that:

$s = sprintf('%02d', $digit);

For more information, refer to the documentation of sprintf.

There's also str_pad

<?php
$input = "Alien";
echo str_pad($input, 10);                      // produces "Alien     "
echo str_pad($input, 10, "-=", STR_PAD_LEFT);  // produces "-=-=-Alien"
echo str_pad($input, 10, "_", STR_PAD_BOTH);   // produces "__Alien___"
echo str_pad($input, 6 , "___");               // produces "Alien_"
?>

Solution using str_pad:

str_pad($digit,2,'0',STR_PAD_LEFT);

Benchmark on php 5.3

Result str_pad : 0.286863088608

Result sprintf : 0.234171152115

Code:

$start = microtime(true);
for ($i=0;$i<100000;$i++) {
    str_pad(9,2,'0',STR_PAD_LEFT);
    str_pad(15,2,'0',STR_PAD_LEFT);
    str_pad(100,2,'0',STR_PAD_LEFT);
}
$end = microtime(true);
echo "Result str_pad : ",($end-$start),"\n";

$start = microtime(true);
for ($i=0;$i<100000;$i++) {
    sprintf("%02d", 9);
    sprintf("%02d", 15);
    sprintf("%02d", 100);
}
$end = microtime(true);
echo "Result sprintf : ",($end-$start),"\n";

The performance of str_pad heavily depends on the length of padding. For more consistent speed you can use str_repeat.

$padded_string = str_repeat("0", $length-strlen($number)) . $number;

Also use string value of the number for better performance.

$number = strval(123);

Tested on PHP 7.4

str_repeat: 0.086055040359497   (number: 123, padding: 1)
str_repeat: 0.085798978805542   (number: 123, padding: 3)
str_repeat: 0.085641145706177   (number: 123, padding: 10)
str_repeat: 0.091305017471313   (number: 123, padding: 100)

str_pad:    0.086184978485107   (number: 123, padding: 1)
str_pad:    0.096981048583984   (number: 123, padding: 3)
str_pad:    0.14874792098999    (number: 123, padding: 10)
str_pad:    0.85979700088501    (number: 123, padding: 100)