<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> <title>Variable scope</title> </head> <body><div class="manualnavbar" style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.variables.predefined.html">Predefined Variables</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.variables.variable.html">Variable variables</a></div> <div class="up"><a href="language.variables.html">Variables</a></div> <div class="home"><a href="index.html">PHP Manual</a></div> </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> </div><hr /><div class="manualnavbar" style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.variables.predefined.html">Predefined Variables</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.variables.variable.html">Variable variables</a></div> <div class="up"><a href="language.variables.html">Variables</a></div> <div class="home"><a href="index.html">PHP Manual</a></div> </div></body></html>