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3.3. Variable Scope

Up to this point, if you don't use functions, any variable you create can be used anywhere in a page. With functions, this is no longer always true. Functions keep their own sets of variables that are distinct from those of the page and of other functions.

The variables defined in a function, including its parameters, are not accessible outside the function, and, by default, variables defined outside a function are not accessible inside the function. The following example illustrates this:

$a = 3;
function foo( ) {
  $a += 2;
foo( );
echo $a;

The variable $a inside the function foo( ) is a different variable than the variable $a outside the variable; even though foo( ) uses the add-and-assign operator, the value of the outer $a remains 3 throughout the life of the page. Inside the function, $a has the value 2.

As we discussed in Chapter 2, the extent to which a variable can be seen in a program is called the scope of the variable. Variables created within a function are inside the scope of the function (i.e., have function-level scope). Variables created outside of functions and objects have global scope and exist anywhere outside of those functions and objects. A few variables provided by PHP have both function-level and global scope.

At first glance, even an experienced programmer may think that in the previous example $a will be 5 by the time the echo statement is reached, so keep that in mind when choosing names for your variables.

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