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</div><hr /><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">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />$a </span><span style="color: #007700">= </span><span style="color: #DD0000">'hello'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="simpara">
A variable variable takes the value of a variable and treats that
as the name of a variable. In the above example,
<em class="emphasis">hello</em>, can be used as the name of a variable
by using two dollar signs. i.e.
</p>
<div class="informalexample">
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">$</span><span style="color: #0000BB">$a </span><span style="color: #007700">= </span><span style="color: #DD0000">'world'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
<p class="simpara">
At this point two variables have been defined and stored in the
PHP symbol tree: <var class="varname"><var class="varname">$a</var></var> with contents "hello" and
<var class="varname"><var class="varname">$hello</var></var> with contents "world". Therefore, this
statement:
</p>
<div class="informalexample">
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">echo </span><span style="color: #DD0000">"</span><span style="color: #0000BB">$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">
<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"><var class="varname">$$a[1]</var></var> then the parser needs to know if you
meant to use <var class="varname"><var class="varname">$a[1]</var></var> as a variable, or if you
wanted <var class="varname"><var class="varname">$$a</var></var> as the variable and then the [1]
index from that variable. The syntax for resolving this ambiguity
is: <var class="varname"><var class="varname">${$a[1]}</var></var> for the first case and
<var class="varname"><var class="varname">${$a}[1]</var></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"><var class="varname">$foo->$bar</var></var>, then the local scope will be examined for
<var class="varname"><var class="varname">$bar</var></var> and its value will be used as the name of the
property of <var class="varname"><var class="varname">$foo</var></var>. This is also true if
<var class="varname"><var class="varname">$bar</var></var> is an array access.
</p>
<div class="caution"><strong class="caution">Caution</strong>
<p class="simpara">
Further dereferencing a variable property that is an array has different
semantics between PHP 5 and PHP 7. The
<a href="migration70.incompatible.html#migration70.incompatible.variable-handling.indirect" class="link">PHP 7.0 migration guide</a>
includes further details on the types of expressions that have changed,
and how to place curly braces to avoid ambiguity.
</p>
</div>
<p class="simpara">
Curly braces may also be used, to clearly delimit the property
name. They are most useful when accessing values within a property that
contains an array, when the property name is made of mulitple parts,
or when the property name contains characters that are not
otherwise valid (e.g. from <span class="function"><a href="function.json-decode.html" class="function">json_decode()</a></span>
or <a href="book.simplexml.html" class="link">SimpleXML</a>).
</p>
<p class="para">
<div class="example" id="example-85">
<p><strong>Example #1 Variable property example</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /></span><span style="color: #007700">class </span><span style="color: #0000BB">foo </span><span style="color: #007700">{<br /> var </span><span style="color: #0000BB">$bar </span><span style="color: #007700">= </span><span style="color: #DD0000">'I am bar.'</span><span style="color: #007700">;<br /> var </span><span style="color: #0000BB">$arr </span><span style="color: #007700">= array(</span><span style="color: #DD0000">'I am A.'</span><span style="color: #007700">, </span><span style="color: #DD0000">'I am B.'</span><span style="color: #007700">, </span><span style="color: #DD0000">'I am C.'</span><span style="color: #007700">);<br /> var </span><span style="color: #0000BB">$r </span><span style="color: #007700">= </span><span style="color: #DD0000">'I am r.'</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">$baz</span><span style="color: #007700">[</span><span style="color: #0000BB">1</span><span style="color: #007700">]} . </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">$start </span><span style="color: #007700">= </span><span style="color: #DD0000">'b'</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">$end </span><span style="color: #007700">= </span><span style="color: #DD0000">'ar'</span><span style="color: #007700">;<br />echo </span><span style="color: #0000BB">$foo</span><span style="color: #007700">->{</span><span style="color: #0000BB">$start </span><span style="color: #007700">. </span><span style="color: #0000BB">$end</span><span style="color: #007700">} . </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">$arr </span><span style="color: #007700">= </span><span style="color: #DD0000">'arr'</span><span style="color: #007700">;<br />echo </span><span style="color: #0000BB">$foo</span><span style="color: #007700">->{</span><span style="color: #0000BB">$arr</span><span style="color: #007700">[</span><span style="color: #0000BB">1</span><span style="color: #007700">]} . </span><span style="color: #DD0000">"\n"</span><span style="color: #007700">;<br /><br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
<div class="example-contents"><p>The above example will output:</p></div>
<div class="example-contents screen"><br />
I am bar.<br />
I am bar.<br />
I am bar.<br />
I am r.<br />
</div>
</div>
</p>
<div class="warning"><strong class="warning">Warning</strong>
<p class="simpara">
Please note that variable variables cannot be used with PHP's
<a href="language.variables.superglobals.html" class="link">Superglobal arrays</a>
within functions or class methods. The variable <em>$this</em>
is also a special variable that cannot be referenced dynamically.
</p>
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