Source of: /manual/en/language.constants.syntax.php
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<div id="language.constants.syntax" class="sect1">
<h2 class="title">Syntax</h2>
<p class="simpara">
You can define a constant by using the
<a href="function.define.php" class="function">define()</a>-function or by using the
<i>const</i> keyword outside a class definition as
of PHP 5.3.0. Once a constant is defined, it can never be
changed or undefined.
</p>
<p class="simpara">
Only scalar data (<a href="language.types.boolean.php" class="type boolean">boolean</a>, <a href="language.types.integer.php" class="type integer">integer</a>,
<a href="language.types.float.php" class="type float">float</a> and <a href="language.types.string.php" class="type string">string</a>) can be contained
in constants. It is possible to define constants as a
<a href="language.types.resource.php" class="type resource">resource</a>, but it should be avoided, as it can cause
unexpected results.
</p>
<p class="simpara">
You can get the value of a constant by simply specifying its name.
Unlike with variables, you should <em class="emphasis">not</em> prepend
a constant with a <i>$</i>.
You can also use the function <a href="function.constant.php" class="function">constant()</a> to
read a constant's value if you wish to obtain the constant's name
dynamically.
Use <a href="function.get-defined-constants.php" class="function">get_defined_constants()</a> to get a list of
all defined constants.
</p>
<blockquote><p><b class="note">Note</b>:
<span class="simpara">
Constants and (global) variables are in a different namespace.
This implies that for example <b><tt class="constant">TRUE</tt></b> and
<var class="varname">$TRUE</var> are generally different.
</span>
</p></blockquote>
<p class="simpara">
If you use an undefined constant, PHP assumes that you mean
the name of the constant itself, just as if you called it as
a <a href="language.types.string.php" class="type string">string</a> (CONSTANT vs "CONSTANT"). An error of level
<a href="ref.errorfunc.php" class="link">E_NOTICE</a> will be issued
when this happens. See also the manual entry on why
<a href="language.types.array.php#language.types.array.foo-bar" class="link">$foo[bar]</a> is
wrong (unless you first <a href="function.define.php" class="function">define()</a>
<i>bar</i> as a constant). If you simply want to check if a
constant is set, use the <a href="function.defined.php" class="function">defined()</a> function.
</p>
<p class="para">
These are the differences between constants and variables:
</p><ul class="itemizedlist">
<li class="listitem">
<span class="simpara">
Constants do not have a dollar sign (<i>$</i>)
before them;
</span>
</li>
<li class="listitem">
<span class="simpara">
Constants may only be defined using the
<a href="function.define.php" class="function">define()</a> function, not by simple assignment;
</span>
</li>
<li class="listitem">
<span class="simpara">
Constants may be defined and accessed anywhere without regard
to variable scoping rules;
</span>
</li>
<li class="listitem">
<span class="simpara">
Constants may not be redefined or undefined once they have been
set; and
</span>
</li>
<li class="listitem">
<span class="simpara">
Constants may only evaluate to scalar values.
</span>
</li>
</ul><p>
</p>
<p class="para">
</p><div class="example">
<p><b>Example #1 Defining Constants</b></p>
<div class="example-contents programlisting">
<div class="phpcode"><code><span style="color: #000000">
<span style="color: #0000BB"><?php<br />define</span><span style="color: #007700">(</span><span style="color: #DD0000">"CONSTANT"</span><span style="color: #007700">, </span><span style="color: #DD0000">"Hello world."</span><span style="color: #007700">);<br />echo </span><span style="color: #0000BB">CONSTANT</span><span style="color: #007700">; </span><span style="color: #FF8000">// outputs "Hello world."<br /></span><span style="color: #007700">echo </span><span style="color: #0000BB">Constant</span><span style="color: #007700">; </span><span style="color: #FF8000">// outputs "Constant" and issues a notice.<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div><p>
</p>
<p class="para">
</p><div class="example">
<p><b>Example #2 Defining Constants using the <i>const</i> keyword</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">// Works as of PHP 5.3.0<br /></span><span style="color: #007700">const </span><span style="color: #0000BB">CONSTANT </span><span style="color: #007700">= </span><span style="color: #DD0000">'Hello World'</span><span style="color: #007700">;<br /><br />echo </span><span style="color: #0000BB">CONSTANT</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div><p>
</p>
<p class="simpara">
See also <a href="language.oop5.constants.php" class="link">Class Constants</a>.
</p>
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