str_pad

since the default pad_type is STR_PAD_RIGHT. using STR_PAD_BOTH were always favor in the right pad if the required number of padding characters can't be evenly divided. 
e.g
<?php
echo str_pad("input", 10, "pp", STR_PAD_BOTH ); // ppinputppp
echo str_pad("input", 6, "p", STR_PAD_BOTH ); // inputp
echo str_pad("input", 8, "p", STR_PAD_BOTH ); //pinputpp
?>

Beware, \str_pad() is NOT able to correctly handle multibyte characters and as \strlen() it is assuming one char == byte. If you have multibyte chars in your string your result string will be shorter than you expect:
<?php
$a = 'áč'; // 2 accented chars
$lenA = \mb_strlen($a);
echo $lenA . PHP_EOL;
$b = \str_pad($a, $lenA + 10, ' ');
$lenB = \mb_strlen($b);
echo $lenB . PHP_EOL;
?>

would produce:
2
10
instead of expected 12. There seem noth to be mb_str_pad() equivalent so you may end you concatenating your string and padding manually:
<?php
$a = 'áč'; // 2 accented chars
$b = mb_str_pad($a, $lenA + 10, ' ');
$lenB = \mb_strlen($b);
echo $lenB . PHP_EOL;
function mb_str_pad(string $str, int $len, string $pad, int $align = \STR_PAD_RIGHT): string
{
 $strLen = \mb_strlen($str);
 if ($strLen >= $len) {
 return $str;
 }
 $diff = $len - $strLen;
 $padding = \mb_substr(\str_repeat($pad, $diff), 0, $diff);
 switch ($align) {
 case \STR_PAD_BOTH:
 $diffHalf = (int)($diff/2 + 0.5);
 $padding = \str_repeat($pad, $diffHalf);
 $result = "{$padding}{$str}{$padding}";
 break;
 case \STR_PAD_LEFT:
 $result = "{$padding}{$str}";
 $result = "{$str}{$padding}";
 break;
 case \STR_PAD_RIGHT:
 default:
 $result = "{$str}{$padding}";
 break;
 }
 return \mb_substr($result, 0, $len);
}
?>

returns expected 12 char long string.

you can use str_pad to display an integer with a fixed amount of digits, like that:
0002
0003
...
0100
by just writing
<?php
 for ($i=0;$i<10000;$i++){
 echo str_pad($i,4,'0',STR_PAD_LEFT)."\n";
 }
?>

i set 4 digits (see parameter #2), but you can set any fitting your needs.

A proper unicode string padder;
<?php
mb_internal_encoding('utf-8'); // @important
function str_pad_unicode($str, $pad_len, $pad_str = ' ', $dir = STR_PAD_RIGHT) {
 $str_len = mb_strlen($str);
 $pad_str_len = mb_strlen($pad_str);
 if (!$str_len && ($dir == STR_PAD_RIGHT || $dir == STR_PAD_LEFT)) {
 $str_len = 1; // @debug
 }
 if (!$pad_len || !$pad_str_len || $pad_len <= $str_len) {
 return $str;
 }
 
 $result = null;
 $repeat = ceil($str_len - $pad_str_len + $pad_len);
 if ($dir == STR_PAD_RIGHT) {
 $result = $str . str_repeat($pad_str, $repeat);
 $result = mb_substr($result, 0, $pad_len);
 } else if ($dir == STR_PAD_LEFT) {
 $result = str_repeat($pad_str, $repeat) . $str;
 $result = mb_substr($result, -$pad_len);
 } else if ($dir == STR_PAD_BOTH) {
 $length = ($pad_len - $str_len) / 2;
 $repeat = ceil($length / $pad_str_len);
 $result = mb_substr(str_repeat($pad_str, $repeat), 0, floor($length)) 
 . $str 
 . mb_substr(str_repeat($pad_str, $repeat), 0, ceil($length));
 }
 
 return $result;
}
?>

Test;
<?php
// needs ie. "test.php" file encoded in "utf-8 without bom"
$s = '...';
for ($i = 3; $i <= 1000; $i++) {
 $s1 = str_pad($s, $i, 'AO', STR_PAD_BOTH); // can not inculde unicode char!!!
 $s2 = str_pad_unicode($s, $i, 'ÄÖ', STR_PAD_BOTH);
 $sl1 = strlen($s1);
 $sl2 = mb_strlen($s2);
 echo "len $sl1: $s1 \n";
 echo "len $sl2: $s2 \n";
 echo "\n";
 if ($sl1 != $sl2) die("Fail!");
}
?>

Output;
len 3: ... 
len 3: ... 
len 4: ...A 
len 4: ...Ä 
len 5: A...A 
len 5: Ä...Ä 
len 6: A...AO 
len 6: Ä...ÄÖ 
...

Incrementing or decrementing numbers in PHP is easy with the ++ and -- operators but it can be difficult to set the precision of the numbers. The str_pad() can be useful for concatenating a string to the beginning or end of the incrementing number to simulate a different precision. 
Good example, we want to increment 001 to 002, 003, 004:
$numbers = [];
for($i = 1; $i <= 4; $i++){
 $numbers[] = str_pad($i, 3, '0', STR_PAD_LEFT);
}
print_r($numbers);
$numbers[0] => '001',
$numbers[1] => '002',
$numbers[2] => '003',
$numbers[3] => '004',
Bad example, we want to increment 001 to 002, 003, 004 but if we set $i = 001 in the for() loop to start with, 001 will be converted to 1 and the incrementing will return: 1, 2, 3, 4 etc... 
$numbers = [];
for($i = 001; $i <= 4; $i++){
 $numbers[] = $i;
}
print_r($numbers);
$numbers[0] => 1,
$numbers[1] => 2,
$numbers[2] => 3,
$numbers[3] => 4,

multibyte version:
<?php
function mb_str_pad($str, $pad_len, $pad_str = ' ', $dir = STR_PAD_RIGHT, $encoding = NULL)
{
 $encoding = $encoding === NULL ? mb_internal_encoding() : $encoding;
 $padBefore = $dir === STR_PAD_BOTH || $dir === STR_PAD_LEFT;
 $padAfter = $dir === STR_PAD_BOTH || $dir === STR_PAD_RIGHT;
 $pad_len -= mb_strlen($str, $encoding);
 $targetLen = $padBefore && $padAfter ? $pad_len / 2 : $pad_len;
 $strToRepeatLen = mb_strlen($pad_str, $encoding);
 $repeatTimes = ceil($targetLen / $strToRepeatLen);
 $repeatedString = str_repeat($pad_str, max(0, $repeatTimes)); // safe if used with valid utf-8 strings
 $before = $padBefore ? mb_substr($repeatedString, 0, floor($targetLen), $encoding) : '';
 $after = $padAfter ? mb_substr($repeatedString, 0, ceil($targetLen), $encoding) : '';
 return $before . $str . $after;
}
?>

In a lot of cases you're better off using str_repeat if you want to use something like - it repeats the entire string.
Using str_repeat, I wrote a full string pad function that should closely mimic str_pad in every other way:
<?php
function full_str_pad($input, $pad_length, $pad_string = '', $pad_type = 0) {
 $str = '';
 $length = $pad_length - strlen($input);
 if ($length > 0) { // str_repeat doesn't like negatives
 if ($pad_type == STR_PAD_RIGHT) { // STR_PAD_RIGHT == 1
 $str = $input.str_repeat($pad_string, $length);
 } elseif ($pad_type == STR_PAD_BOTH) { // STR_PAD_BOTH == 2
 $str = str_repeat($pad_string, floor($length/2));
 $str .= $input;
 $str .= str_repeat($pad_string, ceil($length/2));
 } else { // defaults to STR_PAD_LEFT == 0
 $str = str_repeat($pad_string, $length).$input;
 }
 } else { // if $length is negative or zero we don't need to do anything
 $str = $input;
 }
 return $str;
}
$pad_me = "Test String";
echo '|'.full_str_pad($pad_me, 20, ' ')."|\n";
echo '|'.full_str_pad($pad_me, 20, ' ', STR_PAD_RIGHT)."|\n";
echo '|'.full_str_pad($pad_me, 20, ' ', STR_PAD_BOTH)."|\n";
?>