Why do PHP Array Examples Leave a Trailing Comma?
Because they can. 🙂 The PHP Manual entry for array states:
Having a trailing comma after the last defined array entry, while unusual, is a valid syntax.
Seriously, this is entirely for convenience so you can easily add another element to the array without having to first add the trailing comma to the last entry.
Speaking of other languages: Be careful with this in JavaScript. Some older browsers will throw an error, though newer ones generally allow it.