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</div><hr /><div id="language.constants.syntax" class="sect1">
<h2 class="title">Syntax</h2>
<p class="simpara">
You can define a constant by using the
<span class="function"><a href="function.define.html" class="function">define()</a></span>-function or by using the
<em>const</em> keyword outside a class definition as
of PHP 5.3.0. While <span class="function"><a href="function.define.html" class="function">define()</a></span> allows a constant to be
defined to an arbitrary expression, the <em>const</em> keyword has
restrictions as outlined in the next paragraph.
Once a constant is defined, it can never be
changed or undefined.
</p>
<p class="simpara">
When using the <em>const</em> keyword,
only scalar data (<span class="type"><a href="language.types.boolean.html" class="type boolean">boolean</a></span>, <span class="type"><a href="language.types.integer.html" class="type integer">integer</a></span>,
<span class="type"><a href="language.types.float.html" class="type float">float</a></span> and <span class="type"><a href="language.types.string.html" class="type string">string</a></span>) can be contained
in constants prior to PHP 5.6. From PHP 5.6 onwards, it is possible to
define a constant as a scalar expression, and it is also possible
to define an <span class="type"><a href="language.types.array.html" class="type array">array</a></span> constant. It is possible to define
constants as a <span class="type"><a href="language.types.resource.html" class="type resource">resource</a></span>, 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 <em>$</em>.
You can also use the function <span class="function"><a href="function.constant.html" class="function">constant()</a></span> to
read a constant's value if you wish to obtain the constant's name
dynamically.
Use <span class="function"><a href="function.get-defined-constants.html" class="function">get_defined_constants()</a></span> to get a list of
all defined constants.
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<span class="simpara">
Constants and (global) variables are in a different namespace.
This implies that for example <strong><code>TRUE</code></strong> and
<var class="varname"><var class="varname">$TRUE</var></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 <span class="type"><a href="language.types.string.html" class="type string">string</a></span> (CONSTANT vs "CONSTANT"). An error of level
<a href="ref.errorfunc.html" class="link">E_NOTICE</a> will be issued
when this happens. See also the manual entry on why
<a href="language.types.array.html#language.types.array.foo-bar" class="link">$foo[bar]</a> is
wrong (unless you first <span class="function"><a href="function.define.html" class="function">define()</a></span>
<em>bar</em> as a constant). This does not apply to <a href="language.namespaces.rules.html" class="link">(fully) qualified constants</a>,
which will raise a fatal error if undefined. If you simply want to check if a
constant is set, use the <span class="function"><a href="function.defined.html" class="function">defined()</a></span> function.
</p>
<p class="para">
These are the differences between constants and variables:
<ul class="itemizedlist">
<li class="listitem">
<span class="simpara">
Constants do not have a dollar sign (<em>$</em>)
before them;
</span>
</li>
<li class="listitem">
<span class="simpara">
Prior to PHP 5.3, Constants may only be defined using the
<span class="function"><a href="function.define.html" class="function">define()</a></span> 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. As of PHP 5.6 it is possible
to define array constant using <em>const</em> keywords and as of
PHP 7 array constants can also be defined using <span class="function"><a href="function.define.html" class="function">define()</a></span>
You may use arrays in constant scalar expressions
(for example, <em>const FOO = array(1,2,3)[0];</em>),
but the end result must be a value of allowed type.
</span>
</li>
</ul>
</p>
<p class="para">
<div class="example" id="example-92">
<p><strong>Example #1 Defining Constants</strong></p>
<div class="example-contents">
<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 class="para">
<div class="example" id="example-93">
<p><strong>Example #2 Defining Constants using the <em>const</em> keyword</strong></p>
<div class="example-contents">
<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 /><br /></span><span style="color: #FF8000">// Works as of PHP 5.6.0<br /></span><span style="color: #007700">const </span><span style="color: #0000BB">ANOTHER_CONST </span><span style="color: #007700">= </span><span style="color: #0000BB">CONSTANT</span><span style="color: #007700">.</span><span style="color: #DD0000">'; Goodbye World'</span><span style="color: #007700">;<br />echo </span><span style="color: #0000BB">ANOTHER_CONST</span><span style="color: #007700">;<br /><br />const </span><span style="color: #0000BB">ANIMALS </span><span style="color: #007700">= array(</span><span style="color: #DD0000">'dog'</span><span style="color: #007700">, </span><span style="color: #DD0000">'cat'</span><span style="color: #007700">, </span><span style="color: #DD0000">'bird'</span><span style="color: #007700">);<br />echo </span><span style="color: #0000BB">ANIMALS</span><span style="color: #007700">[</span><span style="color: #0000BB">1</span><span style="color: #007700">]; </span><span style="color: #FF8000">// outputs "cat"<br /><br />// Works as of PHP 7<br /></span><span style="color: #0000BB">define</span><span style="color: #007700">(</span><span style="color: #DD0000">'ANIMALS'</span><span style="color: #007700">, array(<br /> </span><span style="color: #DD0000">'dog'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'cat'</span><span style="color: #007700">,<br /> </span><span style="color: #DD0000">'bird'<br /></span><span style="color: #007700">));<br />echo </span><span style="color: #0000BB">ANIMALS</span><span style="color: #007700">[</span><span style="color: #0000BB">1</span><span style="color: #007700">]; </span><span style="color: #FF8000">// outputs "cat"<br /></span><span style="color: #0000BB">?></span>
</span>
</code></div>
</div>
</div>
</p>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
As opposed to defining constants using <span class="function"><a href="function.define.html" class="function">define()</a></span>,
constants defined using the <em>const</em> keyword must be
declared at the top-level scope because they are defined at compile-time.
This means that they cannot be declared inside functions, loops,
<em>if</em> statements or <em>try</em>/
<em>catch</em> blocks.
</p>
</p></blockquote>
<blockquote class="note"><p><strong class="note">Note</strong>:
<p class="para">
Constants defined using the <em>const</em> keyword are always
case-sensitive, while constants defined using <span class="function"><a href="function.define.html" class="function">define()</a></span>
may be case-insensitive.
</p>
</p></blockquote>
<p class="simpara">
See also <a href="language.oop5.constants.html" class="link">Class Constants</a>.
</p>
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