A variable's scope is the range of the script where it is visible. Variables have either global or local scope. A global variable exists only once in a script, and is visible in every function. Modifications to it in one function are permanent and visible to all functions. Unless declared otherwise, all variables in a script are global. Global variables are useful for values that are relatively constant, or that many functions in the script must access, such as a session id.
A local variable, however, has a limited scope: it exists only within the block that it is declared in. Once that block ends, the variable is destroyed and its values lost. A local variable of the same name declared elsewhere is a different variable. A local variable can even exist multiple times simultaneously, if its block is entered again before it's exited - i.e. a recursive function call. Each call of the function will have a distinct local variable.
Local variables must be explicitly declared, either as parameters to
a script function (here), or with the
statement (here). They are used to clearly pass
parameters to functions, or as temporary "scratch space" for a
function without the side effects of global variable modification.
A local variable with the same name as another in-scope variable
will have precedence over the outer variable. For example, a local
$x has precedence over, and will "hide", a
global variable named
<$x = "fragile"> <!-- global $x -->
<$x = "test"> <!-- only local $x seen here -->
The value of x is $x <!-- back to global $x -->
LOCAL block, references to
$x refer to the
local variable: the global
$x is hidden. Thus, the global
$x value "
fragile" is not lost when the