Variable scope
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:
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:
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.
The global keyword
First, an example use of global:
Example #1 Using global
<?php
$a = 1;
$b = 2;
function Sum()
{
global $a, $b;
$b = $a + $b;
}
Sum();
echo $b;
?>
The above script will output "3". By declaring
$a and $b global within the
function, all references to either variable will refer to the
global version. There is no limit to the number of global
variables that can be manipulated by a function.
A second way to access variables from the global scope is to use
the special PHP-defined $GLOBALS array. The
previous example can be rewritten as:
Example #2 Using $GLOBALS instead of global
<?php
$a = 1;
$b = 2;
function Sum()
{
$GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b'];
}
Sum();
echo $b;
?>
The $GLOBALS array is an associative array with
the name of the global variable being the key and the contents of
that variable being the value of the array element.
Notice how $GLOBALS exists in any scope, this
is because $GLOBALS is a superglobal.
Here's an example demonstrating the power of superglobals:
Example #3 Example demonstrating superglobals and scope
<?php
function test_global()
{
// Most predefined variables aren't "super" and require
// 'global' to be available to the functions local scope.
global $HTTP_POST_VARS;
echo $HTTP_POST_VARS['name'];
// Superglobals are available in any scope and do
// not require 'global'. Superglobals are available
// as of PHP 4.1.0, and HTTP_POST_VARS is now
// deemed deprecated.
echo $_POST['name'];
}
?>
Using static variables
Another important feature of variable scoping is the
static variable. A static variable exists
only in a local function scope, but it does not lose its value
when program execution leaves this scope. Consider the following
example:
Example #4 Example demonstrating need for static variables
<?php
function Test()
{
$a = 0;
echo $a;
$a++;
}
?>
This function is quite useless since every time it is called it
sets $a to 0 and prints
"0". The $a++ which increments the
variable serves no purpose since as soon as the function exits the
$a variable disappears. To make a useful
counting function which will not lose track of the current count,
the $a variable is declared static:
Example #5 Example use of static variables
<?php
function Test()
{
static $a = 0;
echo $a;
$a++;
}
?>
Now, every time the Test() function is called it will print the
value of $a and increment it.
Static variables also provide one way to deal with recursive
functions. A recursive function is one which calls itself. Care
must be taken when writing a recursive function because it is
possible to make it recurse indefinitely. You must make sure you
have an adequate way of terminating the recursion. The following
simple function recursively counts to 10, using the static
variable $count to know when to stop:
Example #6 Static variables with recursive functions
<?php
function Test()
{
static $count = 0;
$count++;
echo $count;
if ($count < 10) {
Test();
}
$count--;
}
?>
Note:
Static variables may be declared as seen in the examples above.
Trying to assign values to these variables which are the
result of expressions will cause a parse error.
Example #7 Declaring static variables
<?php
function foo(){
static $int = 0; // correct
static $int = 1+2; // wrong (as it is an expression)
static $int = sqrt(121); // wrong (as it is an expression too)
$int++;
echo $int;
}
?>
References with global and static variables
The Zend Engine 1, driving PHP 4, implements the
static and
global modifier
for variables in terms of
references. For example, a true global variable
imported inside a function scope with the global
statement actually creates a reference to the global variable. This can
lead to unexpected behaviour which the following example addresses:
Executing this example will result in the following output:
NULL
object(stdClass)(0) {
}
A similar behaviour applies to the static statement.
References are not stored statically:
Executing this example will result in the following output:
Static object: NULL
Static object: NULL
Static object: NULL
Static object: object(stdClass)(1) {
["property"]=>
int(1)
}
This example demonstrates that when assigning a reference to a static
variable, it's not remembered when you call the
&get_instance_ref() function a second time.
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