In PHP 3.0, almost anything you write is an expression. Simple examples that come in mind are constants and variables. If everything works right, this is exactly what will happen. Slightly more complex examples for expressions are functions.
Functions are expressions with the value of their return value.
Usually functions don't just return a static value but compute something. Of course, values in PHP don't have to be integers, and very often they aren't. Each of these value types can be assigned into variables or returned from functions. However, PHP 3 takes expressions much further, in the same way many other languages do. PHP 3 is an expression-oriented language, in the sense that almost everything is an expression.