Source of: /manual/en/function.function-exists.php
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<div id="function.function-exists" class="refentry">
<div class="refnamediv">
<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 <b><tt class="constant">TRUE</tt></b> if the given function has been defined</span></p>
</div>
<a name="function.function-exists.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>function_exists</b></span>
( <span class="methodparam"><span class="type">string</span> <tt class="parameter">$function_name</tt></span>
)</div>
<p class="para rdfs-comment">
Checks the list of defined functions, both built-in (internal) and
user-defined, for <i><tt class="parameter">function_name</tt></i>
.
</p>
</div>
<a name="function.function-exists.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">function_name</tt></i>
</span>
</dt><dd class="listitem">
<p class="para">
The function name, as a string.
</p>
</dd>
</dl>
<p>
</p>
</div>
<a name="function.function-exists.returnvalues"></a><div class="refsect1 returnvalues">
<h3 class="title">Return Values</h3>
<p class="para">
Returns <b><tt class="constant">TRUE</tt></b> if <i><tt class="parameter">function_name</tt></i>
exists and is a
function, <b><tt class="constant">FALSE</tt></b> otherwise.
</p>
<blockquote><p><b class="note">Note</b>:
This function will return <b><tt class="constant">FALSE</tt></b> for constructs, such as
<a href="function.include-once.php" class="function">include_once()</a> and <a href="function.echo.php" class="function">echo()</a>.
<br />
</p></blockquote>
</div>
<a name="function.function-exists.examples"></a><div class="refsect1 examples">
<h3 class="title">Examples</h3>
<p class="para">
</p><div class="example">
<p><b>Example #1 <b>function_exists()</b> 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">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>
</p>
</div>
<a name="function.function-exists.notes"></a><div class="refsect1 notes">
<h3 class="title">Notes</h3>
<blockquote><p><b class="note">Note</b>:
A function name may exist even if the function itself is unusable due to
configuration or compiling options (with the <a href="ref.image.php" class="link">image</a> functions being an example).
<br />
</p></blockquote>
</div>
<a name="function.function-exists.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.method-exists.php" class="function" rel="rdfs-seeAlso">method_exists()</a> - Checks if the class method exists</li>
<li class="member"><a href="function.is-callable.php" class="function" rel="rdfs-seeAlso">is_callable()</a> - Verify that the contents of a variable can be called as a function</li>
<li class="member"><a href="function.get-defined-functions.php" class="function" rel="rdfs-seeAlso">get_defined_functions()</a> - Returns an array of all defined functions</li>
</ul><p>
</p>
</div>
</div><?php manual_footer(); ?>
