I think I've uncovered some errata for this newly purchased book and the form to post errata wasn't working... so I'd like to post it here and see if someone can validate the error.

The code block at the top of page 207 reads:

<?php
$WelcomeMessage = "Hello world";
function translate_greeting($WelcomeMessage)
{
   $WelcomeMessage = "Bonjour Tout Le Monde";
   return $WelcomeMessage;
}
translate_greeting();
echo $WelcomeMessage;
?>

When I run this code, I receive a PHP error: Missing argument 1 for translate_greeting()...

I can fix it by simply changing the function to:
function translate_greeting().

This same error appears on the next page of the book.

Legitimate errata?

<?php
$WelcomeMessage = "Hello world";
function translate_greeting($WelcomeMessage)
{
   $WelcomeMessage = "Bonjour Tout Le Monde";
   return $WelcomeMessage;
}
translate_greeting();
echo $WelcomeMessage;
?>

The reason is simple.
The argument that is used in the function($WelcomeMessage) was not supplied when you called it. You need to modify the line translate_greeting(); so that it looks like this:

translate_greeting($WelcomeMessage);

That should work.