Source of: /manual/en/language.variables.variable.php
<?php
include_once $_SERVER['DOCUMENT_ROOT'] . '/include/shared-manual.inc';
$TOC = array();
$PARENTS = array();
include_once dirname(__FILE__) ."/toc/language.variables.inc";
$setup = array (
'home' =>
array (
0 => 'index.php',
1 => 'PHP Manual',
),
'head' =>
array (
0 => 'UTF-8',
1 => 'en',
),
'this' =>
array (
0 => 'language.variables.variable.php',
1 => 'Variable variables',
),
'up' =>
array (
0 => 'language.variables.php',
1 => 'Variables',
),
'prev' =>
array (
0 => 'language.variables.scope.php',
1 => 'Variable scope',
),
'next' =>
array (
0 => 'language.variables.external.php',
1 => 'Variables From External Sources',
),
);
$setup["toc"] = $TOC;
$setup["parents"] = $PARENTS;
manual_setup($setup);
manual_header();
?>
<div id="language.variables.variable" class="sect1">
<h2 class="title">Variable variables</h2>
<p class="simpara">
Sometimes it is convenient to be able to have variable variable
names. That is, a variable name which can be set and used
dynamically. A normal variable is set with a statement such as:
</p>
<div class="informalexample">
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$a </span><span style="color: #007700">= </span><span style="color: #DD0000">'hello'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="simpara">
A variable variable takes the value of a variable and treats that
as the name of a variable. In the above example,
<em class="emphasis">hello</em>, can be used as the name of a variable
by using two dollar signs. i.e.
</p>
<div class="informalexample">
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">$</span><span style="color: #0000BB">$a </span><span style="color: #007700">= </span><span style="color: #DD0000">'world'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="simpara">
At this point two variables have been defined and stored in the
PHP symbol tree: <var class="varname">$a</var> with contents "hello" and
<var class="varname">$hello</var> with contents "world". Therefore, this
statement:
</p>
<div class="informalexample">
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">echo </span><span style="color: #DD0000">"</span><span style="color: #0000BB">$a</span><span style="color: #DD0000"> </span><span style="color: #007700">${</span><span style="color: #0000BB">$a</span><span style="color: #007700">}</span><span style="color: #DD0000">"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="simpara">
produces the exact same output as:
</p>
<div class="informalexample">
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">echo </span><span style="color: #DD0000">"</span><span style="color: #0000BB">$a</span><span style="color: #DD0000"> </span><span style="color: #0000BB">$hello</span><span style="color: #DD0000">"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="simpara">
i.e. they both produce: <span class="computeroutput">hello world</span>.
</p>
<p class="simpara">
In order to use variable variables with arrays, you have to
resolve an ambiguity problem. That is, if you write
<var class="varname">$$a[1]</var> then the parser needs to know if you
meant to use <var class="varname">$a[1]</var> as a variable, or if you
wanted <var class="varname">$$a</var> as the variable and then the [1]
index from that variable. The syntax for resolving this ambiguity
is: <var class="varname">${$a[1]}</var> for the first case and
<var class="varname">${$a}[1]</var> for the second.
</p>
<p class="simpara">
Class properties may also be accessed using variable property
names. The variable property name will be resolved within the
scope from which the call is made. For instance, if you have an
expression such as <var class="varname">$foo->$bar</var>, then the local
scope will be examined for <var class="varname">$bar</var> and its value
will be used as the name of the property
of <var class="varname">$foo</var>. This is also true
if <var class="varname">$bar</var> is an array access.
</p>
<p class="para">
</p><div class="example">
<p><b>Example #1 Variable function example</b></p>
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">foo </span><span style="color: #007700">{<br /> var </span><span style="color: #0000BB">$bar </span><span style="color: #007700">= </span><span style="color: #DD0000">'I am bar.'</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #0000BB">$foo </span><span style="color: #007700">= new </span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /></span><span style="color: #0000BB">$bar </span><span style="color: #007700">= </span><span style="color: #DD0000">'bar'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$baz </span><span style="color: #007700">= array(</span><span style="color: #DD0000">'foo'</span><span style="color: #007700">, </span><span style="color: #DD0000">'bar'</span><span style="color: #007700">, </span><span style="color: #DD0000">'baz'</span><span style="color: #007700">, </span><span style="color: #DD0000">'quux'</span><span style="color: #007700">);<br />echo </span><span style="color: #0000BB">$foo</span><span style="color: #007700">-></span><span style="color: #0000BB">$bar </span><span style="color: #007700">. </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br />echo </span><span style="color: #0000BB">$foo</span><span style="color: #007700">-></span><span style="color: #0000BB">$baz</span><span style="color: #007700">[</span><span style="color: #0000BB">1</span><span style="color: #007700">] . </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents para"><p>The above example will output:</p></div>
<div class="example-contents screen"><br />
I am bar.<br />
I am bar.<br />
</div>
</div><p>
</p>
<div class="warning"><b class="warning">Warning</b>
<p class="simpara">
Please note that variable variables cannot be used with PHP's
<a href="language.variables.superglobals.php" class="link">Superglobal arrays</a>
within functions or class methods. The variable <i>$this</i>
is also a special variable that cannot be referenced dynamically.
</p>
</div>
</div><?php manual_footer(); ?>
