In another introductory tutorial this morning, phpNoise has their latest tutorial - String Manipulation - Introduction.

The nature of web development requires strings to be created, manipulated and, in some cases, evaluated. Understanding the concept of string manipulation is an important piece for developers.
This introductory tutorial will explain some simple constructs associated to handling strings in addition to some more advanced techniques for those of you who are ambitious.

They cover things like working with quoted strings, escaping things inside strings, and even working with variables inside and around strings. It's quite a good sampling of some of the string functionality of PHP and would be a good read for a lot of new people to the language.