How can PHP be used to convert all uppercase letters to lowercase in a string?

To convert all uppercase letters to lowercase in a string using PHP, you can use the `strtolower()` function. This function takes a string as input and returns a new string with all uppercase letters converted to lowercase. By applying `strtolower()` to the input string, you can easily achieve the desired conversion.

$inputString = "HELLO WORLD";
$outputString = strtolower($inputString);

echo $outputString;