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";
?>

Warning: If your string includes non-ascii characters (eg the British pounds sign), str_pad() will treat these as two characters when calculating the padding.

So for example:
<?php
str_pad($currency_symbol.$showtottopay,12," ",STR_PAD_LEFT);
?>
will produce a different length string depending on whether $currency_symbol is pounds or dollars.

Hope this helps someone -- it caused me a lot of problems with misaligned columns in my invoices until I worked it out.

a different, more robust multibyte version of str_pad that works correctly only if $pad_string is non-multibyte string

function my_mb_str_pad($input, $pad_length, $pad_string=' ', $pad_type=STR_PAD_RIGHT,$encoding='UTF-8'){
    $mb_diff=mb_strlen($str, $encoding)-strlen($string);        
    return str_pad($input,$pad_length+$mb_diff,$pad_string,$pad_type);
}

For me this worked.
$string = 'help';

#First, str_pad() with unique character.
$string = str_pad($string, 10, "*", STR_PAD_BOTH);
#$string = '***help***';

#Second, str_replace with ' '
$string = str_replace("*", " ", $string);

Here is the mcinp's version of mb_str_pad bugfixed: 

<?php
function mb_str_pad($input, $pad_length, $pad_string=' ', $pad_type=STR_PAD_RIGHT,$encoding='UTF-8'){
        $mb_diff=mb_strlen($input, $encoding)-strlen($input);
        return str_pad($input,$pad_length-$mb_diff,$pad_string,$pad_type);
    } 
?> 

Still working correctly only if $pad_string is non-multibyte string

Basically, *all* of you guys have a 'long' way of padding text with html tags (which includes  ) You dont even have to do a str_replace... try the following code and this will work with ANY html tag there is out there and you don't have to worry about tag character lengths so on and so forth:
<?
  $text = "This is pretty interesting!";
  $pad_string = "&nbsp;";
  
  //Pad text on both sides 
  $text = str_pad($text, strlen($text)+(20*strlen($pad_string)), $pad_string, STR_PAD_BOTH);
  print $text." Dont you think?";
?>
Will produce:
          This is pretty interesting!           Dont you think?

Cheers,
Fahad

This is how I pad using   :

str_replace(" ", "  ", str_pad($foo, 10, " ", STR_PAD_LEFT))

Seems to work well using two   tags for each character added, at least for my use. YMMV.

sprintf is faster

$sTime = microtime(true);
$s = sprintf("%'-1000000s", '-');
$eTime = microtime(true);
echo 'sprintf ran in ' . (($eTime - $sTime) * 1000) . ' milliseconds' . "\n";

$sTime = microtime(true);
$s = str_pad('-', 1000000, '-');
$eTime = microtime(true);
echo 'str_pad ran in ' . (($eTime - $sTime) * 1000) . ' milliseconds' . "\n";

//result
sprintf ran in 2.0260810852051 milliseconds
str_pad ran in 26.59797668457 milliseconds

In case you want to pad 2 strings together with a character you can use:

<?php
function pad_between_strings($string1, $string2, $length, $char = " ") {
    $fill_length = $length - ( strlen($string1) + strlen($string2) );
    return $string1 . str_repeat($char, $fill_length) . $string2;
}
?>

For simple padding, you can use sprintf, which is faster:
see http://php.net/sprintf Example #5 "Specifying padding character"

Fills the first argument (mostly a number, f.e. from a <select> loop to display a date or time) with zeroes.

<?php
function zerofill($mStretch, $iLength = 2)
{
    $sPrintfString = '%0' . (int)$iLength . 's';
    return sprintf($sPrintfString, $mStretch);
}
?>

sprintf() is indeed faster than str_pad.

sprintf() is not always faster... It certainly scales a lot better then str_pad so when running a benchmark that pads 10k characters,  sprintf will come out on top. But if you benchmarked a more real world scenario, it seems str_pad comes out the clear winner.

$sTime = microtime(true);
$count = 5;
$s = sprintf("%'\n5s", "\n");
$eTime = microtime(true);
echo 'sprintf ran in ' . (($eTime - $sTime) * 1000) . ' milliseconds' . "\n";

$sTime = microtime(true);
$s = str_pad("\n", 5, "\n");
$eTime = microtime(true);
echo 'str_pad ran in ' . (($eTime - $sTime) * 1000) . ' milliseconds' . "\n";

sprintf ran in 0.015974044799805 milliseconds
str_pad ran in 0.0059604644775391 milliseconds

Here's a quick and simple way to make an mb_str_pad function that works when you have correctly set your internal encoding.

I'm not sure how well this works in all possible scenarios but atleast it worked for me using UTF-8 as internal encoding and using this function on strings containing scandinavian characters "åäöÅÄÖ" that are double byte in UTF-8.

<?php
function mb_str_pad($input, $pad_length, $pad_string=' ', $pad_type=STR_PAD_RIGHT) {
    $diff = strlen($input) - mb_strlen($input);
    return str_pad($input, $pad_length+$diff, $pad_string, $pad_type);
}
?>

<?php
    /**
     * str_pad_html - Pad a string to a certain length with another string.
     * accepts HTML code in param: $strPadString.
     * 
     * @name        str_pad_html()
     * @author        Tim Johannessen <root@it.dk>
     * @version        1.0.0
     * @param        string    $strInput    The array to iterate through, all non-numeric values will be skipped.
     * @param        int    $intPadLength    Padding length, must be greater than zero.
     * @param        string    [$strPadString]    String to pad $strInput with (default: &nbsp;)
     * @param        int        [$intPadType]        STR_PAD_LEFT, STR_PAD_RIGHT (default), STR_PAD_BOTH
     * @return        string    Returns the padded string
    **/
    function str_pad_html($strInput = "", $intPadLength, $strPadString = "&nbsp;", $intPadType = STR_PAD_RIGHT) {
        if (strlen(trim(strip_tags($strInput))) < intval($intPadLength)) {
            
            switch ($intPadType) {
                 // STR_PAD_LEFT
                case 0:
                    $offsetLeft = intval($intPadLength - strlen(trim(strip_tags($strInput))));
                    $offsetRight = 0;
                    break;
                    
                // STR_PAD_RIGHT
                case 1:
                    $offsetLeft = 0;
                    $offsetRight = intval($intPadLength - strlen(trim(strip_tags($strInput))));
                    break;
                    
                // STR_PAD_BOTH
                case 2:
                    $offsetLeft = intval(($intPadLength - strlen(trim(strip_tags($strInput)))) / 2);
                    $offsetRight = round(($intPadLength - strlen(trim(strip_tags($strInput)))) / 2, 0);
                    break;
                    
                // STR_PAD_RIGHT
                default:
                    $offsetLeft = 0;
                    $offsetRight = intval($intPadLength - strlen(trim(strip_tags($strInput))));
                    break;
            }
            
            $strPadded = str_repeat($strPadString, $offsetLeft) . $strInput . str_repeat($strPadString, $offsetRight);
            unset($strInput, $offsetLeft, $offsetRight);
            
            return $strPadded;
        }
        
        else {
            return $strInput;
        }
    }

?>

When provided with a string of characters as the pad value, str_pad uses all the characters as fill, and can leave partial strings. (eg. If the pad value is 'ABC' and it needs 5 characters to pad with, it outputs 'ABCAB'.) This is a problem when you want to pad with non-breaking spaces, the code for which is 6 characters long. 

This can be resolved by first padding the string with a single character that won't be found in the strings such as * then doing a str_replace of * with  .

str_pad() can provide sufficient "zero padding" when using block ciphers and manual padding with openssl_encrypt() and similar.

The example below will pad the 6 character text "Secret" with two \x00 characters and return 8 characters of data. Substitute your plain text and block size as needed.

<?php
$text = "Secret";
$block_size = 8;
$length = ceil(strlen($text) / $block_size) * $block_size;
$data = str_pad($text, $length, "\x00");