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>Variable scope</A
></H1
><P
> 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:
</P
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><PRE
CLASS="programlisting"
> 1
2 $a = 1;
3 include "b.inc";
4 </PRE
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><P
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><P
> 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:
</P
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><PRE
CLASS="programlisting"
> 1
2 $a = 1; /* global scope */
3
4 Function Test () {
5 echo $a; /* reference to local scope variable */
6 }
7
8 Test ();
9 </PRE
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> 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. An example:</P
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><PRE
CLASS="programlisting"
> 1
2 $a = 1;
3 $b = 2;
4
5 Function Sum () {
6 global $a, $b;
7
8 $b = $a + $b;
9 }
10
11 Sum ();
12 echo $b;
13 </PRE
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> 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.
</P
><P
> 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:
</P
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><P
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><PRE
CLASS="programlisting"
> 1
2 $a = 1;
3 $b = 2;
4
5 Function Sum () {
6 $GLOBALS["b"] = $GLOBALS["a"] + $GLOBALS["b"];
7 }
8
9 Sum ();
10 echo $b;
11 </PRE
></TD
></TR
></TABLE
><P
></P
></DIV
><P
> 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.
</P
><P
> Another important feature of variable scoping is the
<I
CLASS="emphasis"
>static</I
> 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:
</P
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><P
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><TR
><TD
><PRE
CLASS="programlisting"
> 1
2 Function Test () {
3 $a = 0;
4 echo $a;
5 $a++;
6 }
7 </PRE
></TD
></TR
></TABLE
><P
></P
></DIV
><P
> 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:</P
><DIV
CLASS="informalexample"
><P
></P
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BORDER="0"
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><TD
><PRE
CLASS="programlisting"
> 1
2 Function Test () {
3 static $a = 0;
4 echo $a;
5 $a++;
6 }
7 </PRE
></TD
></TR
></TABLE
><P
></P
></DIV
><P
> Now, every time the Test() function is called it will print the
value of $a and increment it.
</P
><P
> 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:
</P
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><PRE
CLASS="programlisting"
> 1
2 Function Test () {
3 static $count = 0;
4
5 $count++;
6 echo $count;
7 if ($count < 10) {
8 Test ();
9 }
10 $count--;
11 }
12 </PRE
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