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</div><hr /><div id="functions.user-defined" class="sect1">
<h2 class="title">User-defined functions</h2>
<p class="para">
A function may be defined using syntax such as the following:
</p>
<p class="para">
<div class="example" id="example-150">
<p><strong>Example #1 Pseudo code to demonstrate function uses</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">function </span><span style="color: #0000BB">foo</span><span style="color: #007700">(</span><span style="color: #0000BB">$arg_1</span><span style="color: #007700">, </span><span style="color: #0000BB">$arg_2</span><span style="color: #007700">, </span><span style="color: #FF8000">/* ..., */ </span><span style="color: #0000BB">$arg_n</span><span style="color: #007700">)<br />{<br /> echo </span><span style="color: #DD0000">"Example function.\n"</span><span style="color: #007700">;<br /> return </span><span style="color: #0000BB">$retval</span><span style="color: #007700">;<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="simpara">
Any valid PHP code may appear inside a function, even other
functions and <a href="keyword.class.html" class="link">class</a>
definitions.
</p>
<p class="para">
Function names follow the same rules as other labels in PHP. A
valid function name starts with a letter or underscore, followed
by any number of letters, numbers, or underscores. As a regular
expression, it would be expressed thus:
<em>[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*</em>.
</p>
<div class="tip"><strong class="tip">Tip</strong><p class="simpara">See also the
<a href="userlandnaming.html" class="xref">Userland Naming Guide</a>.</p></div>
<p class="simpara">
Functions need not be defined before they are referenced,
<em class="emphasis">except</em> when a function is conditionally defined as
shown in the two examples below.
</p>
<p class="para">
When a function is defined in a conditional manner such as the two
examples shown. Its definition must be processed <em class="emphasis">prior</em>
to being called.
</p>
<p class="para">
<div class="example" id="example-151">
<p><strong>Example #2 Conditional functions</strong></p>
<div class="example-contents">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br /><br />$makefoo </span><span style="color: #007700">= </span><span style="color: #0000BB">true</span><span style="color: #007700">;<br /><br /></span><span style="color: #FF8000">/* We can't call foo() from here <br /> since it doesn't exist yet,<br /> but we can call bar() */<br /><br /></span><span style="color: #0000BB">bar</span><span style="color: #007700">();<br /><br />if (</span><span style="color: #0000BB">$makefoo</span><span style="color: #007700">) {<br /> function </span><span style="color: #0000BB">foo</span><span style="color: #007700">()<br /> {<br /> echo </span><span style="color: #DD0000">"I don't exist until program execution reaches me.\n"</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #FF8000">/* Now we can safely call foo()<br /> since $makefoo evaluated to true */<br /><br /></span><span style="color: #007700">if (</span><span style="color: #0000BB">$makefoo</span><span style="color: #007700">) </span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /><br />function </span><span style="color: #0000BB">bar</span><span style="color: #007700">() <br />{<br /> echo </span><span style="color: #DD0000">"I exist immediately upon program start.\n"</span><span style="color: #007700">;<br />}<br /><br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="para">
<div class="example" id="example-152">
<p><strong>Example #3 Functions within functions</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">function </span><span style="color: #0000BB">foo</span><span style="color: #007700">() <br />{<br /> function </span><span style="color: #0000BB">bar</span><span style="color: #007700">() <br /> {<br /> echo </span><span style="color: #DD0000">"I don't exist until foo() is called.\n"</span><span style="color: #007700">;<br /> }<br />}<br /><br /></span><span style="color: #FF8000">/* We can't call bar() yet<br /> since it doesn't exist. */<br /><br /></span><span style="color: #0000BB">foo</span><span style="color: #007700">();<br /><br /></span><span style="color: #FF8000">/* Now we can call bar(),<br /> foo()'s processing has<br /> made it accessible. */<br /><br /></span><span style="color: #0000BB">bar</span><span style="color: #007700">();<br /><br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<p class="para">
All functions and classes in PHP have the global scope - they can be
called outside a function even if they were defined inside and vice versa.
</p>
<p class="simpara">
PHP does not support function overloading, nor is it possible to
undefine or redefine previously-declared functions.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<span class="simpara">
Function names are case-insensitive, though it is usually good form
to call functions as they appear in their declaration.
</span>
</p></blockquote>
<p class="simpara">
Both <a href="functions.arguments.html#functions.variable-arg-list" class="link">variable number of
arguments</a> and <a href="functions.arguments.html#functions.arguments.default" class="link">default
arguments</a> are supported in functions. See also the function
references for
<span class="function"><a href="function.func-num-args.html" class="function">func_num_args()</a></span>,
<span class="function"><a href="function.func-get-arg.html" class="function">func_get_arg()</a></span>, and
<span class="function"><a href="function.func-get-args.html" class="function">func_get_args()</a></span> for more information.
</p>
<p class="para">
It is possible to call recursive functions in PHP. However avoid recursive
function/method calls with over 100-200 recursion levels as it can smash
the stack and cause a termination of the current script.
<div class="example" id="example-153">
<p><strong>Example #4 Recursive functions</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">function </span><span style="color: #0000BB">recursion</span><span style="color: #007700">(</span><span style="color: #0000BB">$a</span><span style="color: #007700">)<br />{<br /> if (</span><span style="color: #0000BB">$a </span><span style="color: #007700">< </span><span style="color: #0000BB">20</span><span style="color: #007700">) {<br /> echo </span><span style="color: #DD0000">"</span><span style="color: #0000BB">$a</span><span style="color: #DD0000">\n"</span><span style="color: #007700">;<br /> </span><span style="color: #0000BB">recursion</span><span style="color: #007700">(</span><span style="color: #0000BB">$a </span><span style="color: #007700">+ </span><span style="color: #0000BB">1</span><span style="color: #007700">);<br /> }<br />}<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
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