How to Use strtolower in PHP: Convert Strings to Lowercase

Use the strtolower function in PHP to convert all characters in a string to lowercase. Simply pass the string as an argument like strtolower('Hello'), which returns 'hello'.

Syntax

The strtolower function takes one parameter, a string, and returns a new string with all alphabetic characters converted to lowercase.

• string $string: The input string you want to convert.
• Returns: A string with all uppercase letters changed to lowercase.

php

strtolower(string $string): string

Example

This example shows how to convert a mixed-case string to all lowercase using strtolower.

php

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

Output

hello world!

Common Pitfalls

One common mistake is expecting strtolower to change non-alphabetic characters or handle multibyte characters like accented letters correctly. It only affects ASCII letters A-Z. For multibyte strings, use mb_strtolower.

Also, strtolower does not change the original string but returns a new one, so you must assign or use the returned value.

php

<?php
// Wrong: expecting original string to change
$word = "HELLO";
strtolower($word);
echo $word; // Outputs: HELLO

// Right: assign the result
$word = "HELLO";
$word = strtolower($word);
echo $word; // Outputs: hello
?>

Output

HELLO
hello

Quick Reference

Function	Description	Example	Output
strtolower	Converts ASCII letters to lowercase	strtolower('PHP')	'php'
mb_strtolower	Converts multibyte strings to lowercase	mb_strtolower('ÄÖÜ')	'äöü'

Key Takeaways

Use strtolower to convert ASCII strings to lowercase in PHP.

Always assign the result of strtolower to a variable; it does not modify the original string.

For multibyte or accented characters, use mb_strtolower instead.

strtolower only affects alphabetic characters A-Z, leaving others unchanged.