The scope of a variable is the context within which it is defined. For the most part all PHP variables only have a single scope. This single scope spans included and required files as well. For example:
$a = 1;
Here the $a variable will be available within the included b.inc script. However, within user-defined functions a local function scope is introduced. Any variable used inside a function is by default limited to the local function scope. For example:
$a = 1; /* global scope */
echo $a; /* reference to local scope variable */
This script will not produce any output because the echo statement refers to a local version of the $a variable, and it has not been assigned a value within this scope. You may notice that this is a little bit different from the C language in that global variables in C are automatically available to functions unless specifically overridden by a local definition. This can cause some problems in that people may inadvertently change a global variable. In PHP global variables must be declared global inside a function if they are going to be used in that function.