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</div><hr /><div id="language.variables.scope" class="sect1">
<h2 class="title">Variable scope</h2>
<p class="simpara">
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>
<div class="informalexample">
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$a </span><span style="color: #007700">= </span><span style="color: #0000BB">1</span><span style="color: #007700">;<br />include </span><span style="color: #DD0000">'b.inc'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="simpara">
Here the <var class="varname"><var class="varname">$a</var></var> variable will be available within
the included <var class="filename">b.inc</var> 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>
<div class="informalexample">
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$a </span><span style="color: #007700">= </span><span style="color: #0000BB">1</span><span style="color: #007700">; </span><span style="color: #FF8000">/* global scope */ <br /><br /></span><span style="color: #007700">function </span><span style="color: #0000BB">test</span><span style="color: #007700">()<br />{ <br /> echo </span><span style="color: #0000BB">$a</span><span style="color: #007700">; </span><span style="color: #FF8000">/* reference to local scope variable */ <br /></span><span style="color: #007700">} <br /><br /></span><span style="color: #0000BB">test</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="simpara">
This script will not produce any output because the echo statement
refers to a local version of the <var class="varname"><var class="varname">$a</var></var> variable,
and it has not been assigned a value within this scope. You may
notice that this is a little bit different from the C language in
that global variables in C are automatically available to
functions unless specifically overridden by a local definition.
This can cause some problems in that people may inadvertently
change a global variable. In PHP global variables must be
declared global inside a function if they are going to be used in
that function.
</p>
<div class="sect2" id="language.variables.scope.global">
<h3 class="title">The <em>global</em> keyword</h3>
<p class="simpara">
First, an example use of <em>global</em>:
</p>
<p class="para">
<div class="example" id="example-100">
<p><strong>Example #1 Using <em>global</em></strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$a </span><span style="color: #007700">= </span><span style="color: #0000BB">1</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$b </span><span style="color: #007700">= </span><span style="color: #0000BB">2</span><span style="color: #007700">;<br /><br />function </span><span style="color: #0000BB">Sum</span><span style="color: #007700">()<br />{<br /> global </span><span style="color: #0000BB">$a</span><span style="color: #007700">, </span><span style="color: #0000BB">$b</span><span style="color: #007700">;<br /><br /> </span><span style="color: #0000BB">$b </span><span style="color: #007700">= </span><span style="color: #0000BB">$a </span><span style="color: #007700">+ </span><span style="color: #0000BB">$b</span><span style="color: #007700">;<br />} <br /><br /></span><span style="color: #0000BB">Sum</span><span style="color: #007700">();<br />echo </span><span style="color: #0000BB">$b</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="simpara">
The above script will output <em>3</em>. By declaring
<var class="varname"><var class="varname">$a</var></var> and <var class="varname"><var class="varname">$b</var></var> global within the
function, all references to either variable will refer to the
global version. There is no limit to the number of global
variables that can be manipulated by a function.
</p>
<p class="simpara">
A second way to access variables from the global scope is to use
the special PHP-defined <var class="varname"><var class="varname"><a href="reserved.variables.globals.html" class="classname">$GLOBALS</a></var></var> array. The
previous example can be rewritten as:
</p>
<p class="para">
<div class="example" id="example-101">
<p><strong>Example #2 Using <var class="varname"><var class="varname"><a href="reserved.variables.globals.html" class="classname">$GLOBALS</a></var></var> instead of global</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$a </span><span style="color: #007700">= </span><span style="color: #0000BB">1</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$b </span><span style="color: #007700">= </span><span style="color: #0000BB">2</span><span style="color: #007700">;<br /><br />function </span><span style="color: #0000BB">Sum</span><span style="color: #007700">()<br />{<br /> </span><span style="color: #0000BB">$GLOBALS</span><span style="color: #007700">[</span><span style="color: #DD0000">'b'</span><span style="color: #007700">] = </span><span style="color: #0000BB">$GLOBALS</span><span style="color: #007700">[</span><span style="color: #DD0000">'a'</span><span style="color: #007700">] + </span><span style="color: #0000BB">$GLOBALS</span><span style="color: #007700">[</span><span style="color: #DD0000">'b'</span><span style="color: #007700">];<br />} <br /><br /></span><span style="color: #0000BB">Sum</span><span style="color: #007700">();<br />echo </span><span style="color: #0000BB">$b</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="simpara">
The <var class="varname"><var class="varname"><a href="reserved.variables.globals.html" class="classname">$GLOBALS</a></var></var> 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 <var class="varname"><var class="varname"><a href="reserved.variables.globals.html" class="classname">$GLOBALS</a></var></var> exists in any scope, this
is because <var class="varname"><var class="varname"><a href="reserved.variables.globals.html" class="classname">$GLOBALS</a></var></var> is a <a href="language.variables.superglobals.html" class="link">superglobal</a>.
Here's an example demonstrating the power of superglobals:
</p>
<p class="para">
<div class="example" id="example-102">
<p><strong>Example #3 Example demonstrating superglobals and scope</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">test_global</span><span style="color: #007700">()<br />{<br /> </span><span style="color: #FF8000">// Most predefined variables aren't "super" and require <br /> // 'global' to be available to the functions local scope.<br /> </span><span style="color: #007700">global </span><span style="color: #0000BB">$HTTP_POST_VARS</span><span style="color: #007700">;<br /> <br /> echo </span><span style="color: #0000BB">$HTTP_POST_VARS</span><span style="color: #007700">[</span><span style="color: #DD0000">'name'</span><span style="color: #007700">];<br /> <br /> </span><span style="color: #FF8000">// Superglobals are available in any scope and do <br /> // not require 'global'. Superglobals are available <br /> // as of PHP 4.1.0, and HTTP_POST_VARS is now<br /> // deemed deprecated.<br /> </span><span style="color: #007700">echo </span><span style="color: #0000BB">$_POST</span><span style="color: #007700">[</span><span style="color: #DD0000">'name'</span><span style="color: #007700">];<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Using <em>global</em> keyword outside a function is not an
error. It can be used if the file is included from inside a function.
</p>
</p></blockquote>
</div>
<div class="sect2" id="language.variables.scope.static">
<h3 class="title">Using <em>static</em> variables</h3>
<p class="simpara">
Another important feature of variable scoping is the
<em class="emphasis">static</em> variable. A static variable exists
only in a local function scope, but it does not lose its value
when program execution leaves this scope. Consider the following
example:
</p>
<p class="para">
<div class="example" id="example-103">
<p><strong>Example #4 Example demonstrating need for static variables</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">test</span><span style="color: #007700">()<br />{<br /> </span><span style="color: #0000BB">$a </span><span style="color: #007700">= </span><span style="color: #0000BB">0</span><span style="color: #007700">;<br /> echo </span><span style="color: #0000BB">$a</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">$a</span><span style="color: #007700">++;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="simpara">
This function is quite useless since every time it is called it
sets <var class="varname"><var class="varname">$a</var></var> to <em>0</em> and prints
<em>0</em>. The <var class="varname"><var class="varname">$a</var></var>++ which increments the
variable serves no purpose since as soon as the function exits the
<var class="varname"><var class="varname">$a</var></var> variable disappears. To make a useful
counting function which will not lose track of the current count,
the <var class="varname"><var class="varname">$a</var></var> variable is declared static:
</p>
<p class="para">
<div class="example" id="example-104">
<p><strong>Example #5 Example use of static variables</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">test</span><span style="color: #007700">()<br />{<br /> static </span><span style="color: #0000BB">$a </span><span style="color: #007700">= </span><span style="color: #0000BB">0</span><span style="color: #007700">;<br /> echo </span><span style="color: #0000BB">$a</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">$a</span><span style="color: #007700">++;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="simpara">
Now, <var class="varname"><var class="varname">$a</var></var> is initialized only in first call of function
and every time the <em>test()</em> function is called it will print the
value of <var class="varname"><var class="varname">$a</var></var> and increment it.
</p>
<p class="simpara">
Static variables also provide one way to deal with recursive
functions. A recursive function is one which calls itself. Care
must be taken when writing a recursive function because it is
possible to make it recurse indefinitely. You must make sure you
have an adequate way of terminating the recursion. The following
simple function recursively counts to 10, using the static
variable <var class="varname"><var class="varname">$count</var></var> to know when to stop:
</p>
<p class="para">
<div class="example" id="example-105">
<p><strong>Example #6 Static variables with recursive functions</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">test</span><span style="color: #007700">()<br />{<br /> static </span><span style="color: #0000BB">$count </span><span style="color: #007700">= </span><span style="color: #0000BB">0</span><span style="color: #007700">;<br /><br /> </span><span style="color: #0000BB">$count</span><span style="color: #007700">++;<br /> echo </span><span style="color: #0000BB">$count</span><span style="color: #007700">;<br /> if (</span><span style="color: #0000BB">$count </span><span style="color: #007700">< </span><span style="color: #0000BB">10</span><span style="color: #007700">) {<br /> </span><span style="color: #0000BB">test</span><span style="color: #007700">();<br /> }<br /> </span><span style="color: #0000BB">$count</span><span style="color: #007700">--;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Static variables may be declared as seen in the examples above.
Trying to assign values to these variables which are the
result of expressions will cause a parse error.
</p>
<p class="para">
<div class="example" id="example-106">
<p><strong>Example #7 Declaring static variables</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">foo</span><span style="color: #007700">(){<br /> static </span><span style="color: #0000BB">$int </span><span style="color: #007700">= </span><span style="color: #0000BB">0</span><span style="color: #007700">; </span><span style="color: #FF8000">// correct <br /> </span><span style="color: #007700">static </span><span style="color: #0000BB">$int </span><span style="color: #007700">= </span><span style="color: #0000BB">1</span><span style="color: #007700">+</span><span style="color: #0000BB">2</span><span style="color: #007700">; </span><span style="color: #FF8000">// wrong (as it is an expression)<br /> </span><span style="color: #007700">static </span><span style="color: #0000BB">$int </span><span style="color: #007700">= </span><span style="color: #0000BB">sqrt</span><span style="color: #007700">(</span><span style="color: #0000BB">121</span><span style="color: #007700">); </span><span style="color: #FF8000">// wrong (as it is an expression too)<br /><br /> </span><span style="color: #0000BB">$int</span><span style="color: #007700">++;<br /> echo </span><span style="color: #0000BB">$int</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</p></blockquote>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Static declarations are resolved in compile-time.
</p>
</p></blockquote>
</div>
<div class="sect2" id="language.variables.scope.references">
<h3 class="title">References with <em>global</em> and <em>static</em> variables</h3>
<p class="simpara">
The Zend Engine 1, driving PHP 4, implements the
<a href="language.variables.scope.html#language.variables.scope.static" class="link">static</a> and
<a href="language.variables.scope.html#language.variables.scope.global" class="link">global</a> modifier
for variables in terms of <a href="language.references.html" class="link">
references</a>. For example, a true global variable
imported inside a function scope with the <em>global</em>
statement actually creates a reference to the global variable. This can
lead to unexpected behaviour which the following example addresses:
</p>
<div class="informalexample">
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function </span><span style="color: #0000BB">test_global_ref</span><span style="color: #007700">() {<br /> global </span><span style="color: #0000BB">$obj</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">$obj </span><span style="color: #007700">= &new </span><span style="color: #0000BB">stdclass</span><span style="color: #007700">;<br />}<br /><br />function </span><span style="color: #0000BB">test_global_noref</span><span style="color: #007700">() {<br /> global </span><span style="color: #0000BB">$obj</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">$obj </span><span style="color: #007700">= new </span><span style="color: #0000BB">stdclass</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #0000BB">test_global_ref</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$obj</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">test_global_noref</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$obj</span><span style="color: #007700">);<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="para">The above example will output:</p>
<div class="example-contents screen"><br />
NULL<br />
object(stdClass)(0) {<br />
}<br />
</div>
<p class="simpara">
A similar behaviour applies to the <em>static</em> statement.
References are not stored statically:
</p>
<div class="informalexample">
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">function &</span><span style="color: #0000BB">get_instance_ref</span><span style="color: #007700">() {<br /> static </span><span style="color: #0000BB">$obj</span><span style="color: #007700">;<br /><br /> echo </span><span style="color: #DD0000">'Static object: '</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$obj</span><span style="color: #007700">);<br /> if (!isset(</span><span style="color: #0000BB">$obj</span><span style="color: #007700">)) {<br /> </span><span style="color: #FF8000">// Assign a reference to the static variable<br /> </span><span style="color: #0000BB">$obj </span><span style="color: #007700">= &new </span><span style="color: #0000BB">stdclass</span><span style="color: #007700">;<br /> }<br /> </span><span style="color: #0000BB">$obj</span><span style="color: #007700">-></span><span style="color: #0000BB">property</span><span style="color: #007700">++;<br /> return </span><span style="color: #0000BB">$obj</span><span style="color: #007700">;<br />}<br /><br />function &</span><span style="color: #0000BB">get_instance_noref</span><span style="color: #007700">() {<br /> static </span><span style="color: #0000BB">$obj</span><span style="color: #007700">;<br /><br /> echo </span><span style="color: #DD0000">'Static object: '</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">var_dump</span><span style="color: #007700">(</span><span style="color: #0000BB">$obj</span><span style="color: #007700">);<br /> if (!isset(</span><span style="color: #0000BB">$obj</span><span style="color: #007700">)) {<br /> </span><span style="color: #FF8000">// Assign the object to the static variable<br /> </span><span style="color: #0000BB">$obj </span><span style="color: #007700">= new </span><span style="color: #0000BB">stdclass</span><span style="color: #007700">;<br /> }<br /> </span><span style="color: #0000BB">$obj</span><span style="color: #007700">-></span><span style="color: #0000BB">property</span><span style="color: #007700">++;<br /> return </span><span style="color: #0000BB">$obj</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #0000BB">$obj1 </span><span style="color: #007700">= </span><span style="color: #0000BB">get_instance_ref</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">$still_obj1 </span><span style="color: #007700">= </span><span style="color: #0000BB">get_instance_ref</span><span style="color: #007700">();<br />echo </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$obj2 </span><span style="color: #007700">= </span><span style="color: #0000BB">get_instance_noref</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">$still_obj2 </span><span style="color: #007700">= </span><span style="color: #0000BB">get_instance_noref</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="para">The above example will output:</p>
<div class="example-contents screen"><br />
Static object: NULL<br />
Static object: NULL<br />
<br />
Static object: NULL<br />
Static object: object(stdClass)(1) {<br />
["property"]=><br />
int(1)<br />
}<br />
</div>
<p class="simpara">
This example demonstrates that when assigning a reference to a static
variable, it's not <em class="emphasis">remembered</em> when you call the
<em>&get_instance_ref()</em> function a second time.
</p>
</div>
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