Source of: /manual/en/function.defined.php
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<div id="function.defined" class="refentry">
<div class="refnamediv">
<h1 class="refname">defined</h1>
<p class="verinfo">(PHP 4, PHP 5)</p><p class="refpurpose"><span class="refname">defined</span> — <span class="dc-title">Checks whether a given named constant exists</span></p>
</div>
<a name="function.defined.description"></a><div class="refsect1 description">
<h3 class="title">Description</h3>
<div class="methodsynopsis dc-description">
<span class="type">bool</span> <span class="methodname"><b>defined</b></span>
( <span class="methodparam"><span class="type">string</span> <tt class="parameter">$name</tt></span>
)</div>
<p class="para rdfs-comment">
Checks whether the given constant exists and is defined.
</p>
<blockquote><p><b class="note">Note</b>:
If you want to see if a variable exists, use <a href="function.isset.php" class="function">isset()</a>
as <b>defined()</b> only applies to <a href="language.constants.php" class="link">constants</a>. If you want to see if a
function exists, use <a href="function.function-exists.php" class="function">function_exists()</a>.
<br />
</p></blockquote>
</div>
<a name="function.defined.parameters"></a><div class="refsect1 parameters">
<h3 class="title">Parameters</h3>
<p class="para">
</p><dl>
<dt class="varlistentry">
<span class="term"><i><tt class="parameter">name</tt></i>
</span>
</dt><dd class="listitem">
<p class="para">
The constant name.
</p>
</dd>
</dl>
<p>
</p>
</div>
<a name="function.defined.returnvalues"></a><div class="refsect1 returnvalues">
<h3 class="title">Return Values</h3>
<p class="para">
Returns <b><tt class="constant">TRUE</tt></b> if the named constant given by <i><tt class="parameter">name</tt></i>
has been defined, <b><tt class="constant">FALSE</tt></b> otherwise.
</p>
</div>
<a name="function.defined.examples"></a><div class="refsect1 examples">
<h3 class="title">Examples</h3>
<p class="para">
</p><div class="example">
<p><b>Example #1 Checking Constants</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: #FF8000">/* Note the use of quotes, this is important. This example is checking<br /> * if the string 'TEST' is the name of a constant named TEST */<br /></span><span style="color: #007700">if (</span><span style="color: #0000BB">defined</span><span style="color: #007700">(</span><span style="color: #DD0000">'TEST'</span><span style="color: #007700">)) {<br /> echo </span><span style="color: #0000BB">TEST</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div><p>
</p>
</div>
<a name="function.defined.seealso"></a><div class="refsect1 seealso">
<h3 class="title">See Also</h3>
<p class="para">
</p><ul class="simplelist">
<li class="member"><a href="function.define.php" class="function" rel="rdfs-seeAlso">define()</a> - Defines a named constant</li>
<li class="member"><a href="function.constant.php" class="function" rel="rdfs-seeAlso">constant()</a> - Returns the value of a constant</li>
<li class="member"><a href="function.get-defined-constants.php" class="function" rel="rdfs-seeAlso">get_defined_constants()</a> - Returns an associative array with the names of all the constants and their values</li>
<li class="member"><a href="function.function-exists.php" class="function" rel="rdfs-seeAlso">function_exists()</a> - Return TRUE if the given function has been defined</li>
<li class="member">The section on <a href="language.constants.php" class="link">Constants</a></li>
</ul><p>
</p>
</div>
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