strtolower() / strtoupper() / mb_strtolower() Since: PHP 4（2000）

// Converts all letters to lowercase.
strtolower($string);

// Converts all letters to uppercase.
strtoupper($string);

// Capitalizes only the first character of the string.
ucfirst($string);

// Capitalizes the first letter of each word.
ucwords($string, $separators);

// Converts to lowercase with multibyte character support.
mb_strtolower($string, $encoding);

// Converts to uppercase with multibyte character support.
mb_strtoupper($string, $encoding);

Returns the converted string. The original string is not modified.

<?php
// Converts all letters to lowercase.
echo strtolower("Hello WORLD"); // Outputs "hello world".

// Converts all letters to uppercase.
echo strtoupper("Hello World"); // Outputs "HELLO WORLD".

// Capitalizes only the first character.
echo ucfirst("hello world"); // Outputs "Hello world".

// Capitalizes the first letter of each word.
echo ucwords("hello world php"); // Outputs "Hello World Php".

// Specifies a custom delimiter to split words.
echo ucwords("hello-world-php", "-"); // Outputs "Hello-World-Php".

// Useful for case-insensitive comparisons.
$input = "Tokyo";
if (strtolower($input) === "tokyo") {
	echo "Matched."; // Compares strings without regard to case.
}

// Multibyte-aware conversion.
echo mb_strtoupper("straße", "UTF-8"); // Outputs "STRASSE". The German eszett is converted.

// Example: normalizing an email address.
$email = "User@Example.COM";
echo strtolower($email); // Outputs "user@example.com".

strtolower() and strtoupper() convert the case of all alphabetic characters in a string at once. They are commonly used for normalizing email addresses and URLs, and for performing case-insensitive comparisons.

ucfirst() is handy when you want to capitalize only the first letter of a sentence, while ucwords() is useful for converting text to title case. ucwords() accepts a second argument to specify custom delimiters, making it suitable for strings separated by hyphens, underscores, or other characters.

Use mb_strtolower() and mb_strtoupper() when you need to handle multibyte characters such as the German eszett. For string searching, strpos() is useful, and for case-insensitive replacement, str_ireplace() is a good choice.