What are the methods to convert the first letter of a string to uppercase or lowercase in PHP?

Convert the first letter of each word to uppercase: ucwords()

<?php
$foo = &#39;hello world!&#39;;
$foo = ucwords($foo);             // Hello World!
 
$bar = &#39;HELLO WORLD!&#39;;
$bar = ucwords($bar);             // HELLO WORLD!
$bar = ucwords(strtolower($bar)); // Hello World!
?>

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The first letter of the first word becomes uppercase: ucfirst()

<?php
$foo = &#39;hello world!&#39;;
$foo = ucfirst($foo);             // Hello world!
 
$bar = &#39;HELLO WORLD!&#39;;
$bar = ucfirst($bar);             // HELLO WORLD!
$bar = ucfirst(strtolower($bar)); // Hello world!
?>

The first letter of the first word becomes lowercase: lcfirst()

<?php
$foo = &#39;HelloWorld&#39;;
$foo = lcfirst($foo);             // helloWorld
 
$bar = &#39;HELLO WORLD!&#39;;
$bar = lcfirst($bar);             // hELLO WORLD!
$bar = lcfirst(strtoupper($bar)); // hELLO WORLD!
?>

Additional knowledge points:

Change all letters to uppercase: strtoupper()

Change all letters to lowercase: strtolower()

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