PHP How to Convert String to Lowercase

Use the built-in PHP function strtolower() to convert a string to lowercase, like strtolower('Hello') which returns 'hello'.

Examples

InputHELLO

Outputhello

InputPhp Is Fun!

Outputphp is fun!

Input123 ABC xyz!

Output123 abc xyz!

How to Think About It

To convert a string to lowercase in PHP, think about changing every uppercase letter to its lowercase form while leaving other characters unchanged. PHP provides a simple function that does this for you, so you just need to pass your string to it.

Algorithm

1

Get the input string.

2

Use the built-in function to convert all uppercase letters to lowercase.

3

Return or print the converted lowercase string.

Code

php

<?php
$input = "Hello World!";
$lowercase = strtolower($input);
echo $lowercase;
?>

Output

hello world!

Dry Run

Let's trace converting 'Hello World!' to lowercase using strtolower()

1

Input string

The input string is 'Hello World!'

2

Convert to lowercase

strtolower('Hello World!') changes all uppercase letters to lowercase

3

Output result

The output is 'hello world!'

Original	Lowercase
Hello World!	hello world!

Why This Works

Step 1: Function Purpose

The strtolower() function changes all uppercase letters in a string to lowercase.

Step 2: Input Handling

It processes each character and converts only uppercase letters, leaving numbers and symbols unchanged.

Step 3: Output

It returns a new string with all letters in lowercase, which you can print or use further.

Alternative Approaches

mb_strtolower()

php

<?php
$input = "Äpfel";
$lowercase = mb_strtolower($input, 'UTF-8');
echo $lowercase;
?>

Use <code>mb_strtolower()</code> for multibyte (UTF-8) strings to correctly convert accented or non-ASCII characters.

Complexity: O(n) time, O(n) space

Time Complexity

The function processes each character once, so the time grows linearly with the string length.

Space Complexity

A new string is created for the lowercase result, so space also grows linearly with input size.

Which Approach is Fastest?

strtolower() is fastest for ASCII strings; mb_strtolower() is needed for UTF-8 but slightly slower due to extra encoding handling.

Approach	Time	Space	Best For
strtolower()	O(n)	O(n)	Simple ASCII strings
mb_strtolower()	O(n)	O(n)	UTF-8 strings with special characters