PHP str_pad: How to Pad a String to a Certain Length

In PHP, formatting strings to a specific length is a common requirement for aligning text, generating serial numbers, or creating fixed-width data files. This article explains how to use the built-in str_pad() function to pad a string with other characters until it reaches a desired length. You will learn the function’s syntax, its parameters, and practical examples of padding to the left, right, and both sides of a string.

The str_pad() Function Syntax

The str_pad() function takes an input string and pads it to a new length. The basic syntax is:

str_pad(
    string $string, 
    int $length, 
    string $pad_string = " ", 
    int $pad_type = STR_PAD_RIGHT
): string

Parameters

• $string: The original input string you want to pad.
• $length: The target length of the final string. If this value is less than or equal to the length of the input string, no padding takes place, and the original string is returned.
• $pad_string: The character or characters to use for padding. The default is a single space " ".
• $pad_type: Determines where to add the padding. It accepts one of three PHP constants:
  • STR_PAD_RIGHT (Default): Adds padding to the right end of the string.
  • STR_PAD_LEFT: Adds padding to the left end of the string (useful for leading zeros).
  • STR_PAD_BOTH: Distributes padding evenly to both sides. If the padding count is odd, the right side gets the extra padding.

Practical Examples

1. Pad a String to the Right (Default)

By default, str_pad() adds spaces to the right side of the string.

$string = "Hello";
$result = str_pad($string, 10);

echo "'$result'"; 
// Output: 'Hello     ' (5 spaces added to the right)

You can also specify a custom padding character, such as a dot:

$string = "Hello";
$result = str_pad($string, 10, ".");

echo $result; 
// Output: Hello.....

2. Pad a String to the Left (Leading Zeros)

To pad a string to the left, use the STR_PAD_LEFT constant. This is highly useful for formatting numbers, invoice IDs, or dates with leading zeros.

$invoice_id = "42";
$padded_id = str_pad($invoice_id, 6, "0", STR_PAD_LEFT);

echo $padded_id; 
// Output: 000042

3. Pad Both Sides (Centering Text)

To center-align a string within a block of padding, use STR_PAD_BOTH.

$title = "Title";
$padded_title = str_pad($title, 11, "-", STR_PAD_BOTH);

echo $padded_title; 
// Output: ---Title---

4. Using Multiple Characters for Padding

If your $pad_string is longer than one character, the function will repeat it as necessary, truncating the last repetition if it exceeds the target length.

$string = "PHP";
$result = str_pad($string, 10, "123");

echo $result; 
// Output: PHP1231231