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<title>Return TRUE if the given function has been defined</title>
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<h1 class="refname">function_exists</h1>
<p class="verinfo">(PHP 4, PHP 5)</p><p class="refpurpose"><span class="refname">function_exists</span> — <span class="dc-title">Return <strong><code>TRUE</code></strong> if the given function has been defined</span></p>
</div>
<div class="refsect1 description" id="refsect1-function.function-exists-description">
<h3 class="title">Description</h3>
<div class="methodsynopsis dc-description">
<span class="type">bool</span> <span class="methodname"><strong>function_exists</strong></span>
( <span class="methodparam"><span class="type">string</span> <code class="parameter">$function_name</code></span>
)</div>
<p class="para rdfs-comment">
Checks the list of defined functions, both built-in (internal) and
user-defined, for <em><code class="parameter">function_name</code></em>.
</p>
</div>
<div class="refsect1 parameters" id="refsect1-function.function-exists-parameters">
<h3 class="title">Parameters</h3>
<p class="para">
<dl>
<dt>
<span class="term"><em><code class="parameter">function_name</code></em></span>
<dd>
<p class="para">
The function name, as a string.
</p>
</dd>
</dt>
</dl>
</p>
</div>
<div class="refsect1 returnvalues" id="refsect1-function.function-exists-returnvalues">
<h3 class="title">Return Values</h3>
<p class="para">
Returns <strong><code>TRUE</code></strong> if <em><code class="parameter">function_name</code></em> exists and is a
function, <strong><code>FALSE</code></strong> otherwise.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
This function will return <strong><code>FALSE</code></strong> for constructs, such as
<span class="function"><a href="function.include-once.html" class="function">include_once</a></span> and <span class="function"><a href="function.echo.html" class="function">echo</a></span>.
</p>
</p></blockquote>
</div>
<div class="refsect1 examples" id="refsect1-function.function-exists-examples">
<h3 class="title">Examples</h3>
<p class="para">
<div class="example" id="example-5116">
<p><strong>Example #1 <span class="function"><strong>function_exists()</strong></span> 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">if (</span><span style="color: #0000BB">function_exists</span><span style="color: #007700">(</span><span style="color: #DD0000">'imap_open'</span><span style="color: #007700">)) {<br /> echo </span><span style="color: #DD0000">"IMAP functions are available.<br />\n"</span><span style="color: #007700">;<br />} else {<br /> echo </span><span style="color: #DD0000">"IMAP functions are not available.<br />\n"</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
</div>
<div class="refsect1 notes" id="refsect1-function.function-exists-notes">
<h3 class="title">Notes</h3>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
A function name may exist even if the function itself is unusable due to
configuration or compiling options (with the <a href="ref.image.html" class="link">image</a> functions being an example).
</p>
</p></blockquote>
</div>
<div class="refsect1 seealso" id="refsect1-function.function-exists-seealso">
<h3 class="title">See Also</h3>
<p class="para">
<ul class="simplelist">
<li class="member"> <span class="function"><a href="function.method-exists.html" class="function" rel="rdfs-seeAlso">method_exists()</a> - Checks if the class method exists</span></li>
<li class="member"> <span class="function"><a href="function.is-callable.html" class="function" rel="rdfs-seeAlso">is_callable()</a> - Verify that the contents of a variable can be called as a function</span></li>
<li class="member"> <span class="function"><a href="function.get-defined-functions.html" class="function" rel="rdfs-seeAlso">get_defined_functions()</a> - Returns an array of all defined functions</span></li>
<li class="member"> <span class="function"><a href="function.class-exists.html" class="function" rel="rdfs-seeAlso">class_exists()</a> - Checks if the class has been defined</span></li>
<li class="member"> <span class="function"><a href="function.extension-loaded.html" class="function" rel="rdfs-seeAlso">extension_loaded()</a> - Find out whether an extension is loaded</span></li>
</ul>
</p>
</div>
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