Core PHP Programming, Third Edition

4.3. Scope

In order to avoid clashes between variables in different functions, PHP includes the notion of scope. Each line of code belongs to a certain scope. Code that appears inside a function is considered to belong to the function's scope. Code that appears outside of any function is considered to belong to the global scope. The scope is the property that determines which memory table is used for storing variables and in turn which variables are accessible.

Variables declared inside a function scope are local variables. Local variables are the private property of a function and may never be seen or manipulated outside the scope of the function. Variables used outside the scope of any function are global variables. Unlike some other languages, ...