A printable version of the manual in pdf format is available from php.net. Constants Operators Chapter 9. In PHP, almost anything you write is an expression. The most basic forms of expressions are constants and variables. Slightly more complex examples for expressions are functions.
Functions are expressions with the value of their return value.
Of course, values in PHP don't have to be integers, and very often they aren't. However, PHP takes expressions much further, in the same way many other languages do. PHP is an expression-oriented language, in the sense that almost everything is an expression. But the truth is that there's one additional value involved here, and that's the value of the assignment itself. The assignment itself evaluates to the assigned value, in this case 5.