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    <p>As of PHP 5.3 there are two ways to <a href="http://php.net/language.constants.syntax" rel="noreferrer">define constants</a>: Either using the <code>const</code> keyword or using the <a href="http://php.net/define" rel="noreferrer"><code>define()</code></a> function:</p> <pre><code>const FOO = 'BAR'; define('FOO', 'BAR'); </code></pre> <p>The fundamental difference between those two ways is that <code>const</code> defines constants at compile time, whereas <code>define</code> defines them at run time. This causes most of <code>const</code>'s disadvantages. Some disadvantages of <code>const</code> are:</p> <ul> <li><p><code>const</code> cannot be used to conditionally define constants. To define a global constant, it has to be used in the outermost scope:</p> <pre><code>if (...) { const FOO = 'BAR'; // invalid } // but if (...) { define('FOO', 'BAR'); // valid } </code></pre> <p>Why would you want to do that anyways? One common application is to check whether the constant is already defined:</p> <pre><code>if (!defined('FOO')) { define('FOO', 'BAR'); } </code></pre></li> <li><p><code>const</code> accepts a static scalar (number, string or other constant like <code>true</code>, <code>false</code>, <code>null</code>, <code>__FILE__</code>), whereas <code>define()</code> takes any expression. Since PHP 5.6 constant expressions are allowed in <code>const</code> as well:</p> <pre><code>const BIT_5 = 1 &lt;&lt; 5; // valid since PHP 5.6, invalid previously define('BIT_5', 1 &lt;&lt; 5); // always valid </code></pre></li> <li><p><code>const</code> takes a plain constant name, whereas <code>define()</code> accepts any expression as name. This allows to do things like this:</p> <pre><code>for ($i = 0; $i &lt; 32; ++$i) { define('BIT_' . $i, 1 &lt;&lt; $i); } </code></pre></li> <li><p><code>const</code>s are always case sensitive, whereas <code>define()</code> allows you to define case insensitive constants by passing <code>true</code> as the third argument:</p> <pre><code>define('FOO', 'BAR', true); echo FOO; // BAR echo foo; // BAR </code></pre></li> </ul> <p>So, that was the bad side of things. Now let's look at the reason why I personally always use <code>const</code> unless one of the above situations occurs:</p> <ul> <li><code>const</code> simply reads nicer. It's a language construct instead of a function and also is consistent with how you define constants in classes.</li> <li><code>const</code>, being a language construct, can be statically analysed by automated tooling.</li> <li><p><code>const</code> defines a constant in the current namespace, while <code>define()</code> has to be passed the full namespace name:</p> <pre><code>namespace A\B\C; // To define the constant A\B\C\FOO: const FOO = 'BAR'; define('A\B\C\FOO', 'BAR'); </code></pre></li> <li><p>Since PHP 5.6 <code>const</code> constants can also be arrays, while <code>define()</code> does not support arrays yet. However arrays will be supported for both cases in PHP 7.</p> <pre><code>const FOO = [1, 2, 3]; // valid in PHP 5.6 define('FOO', [1, 2, 3]); // invalid in PHP 5.6, valid in PHP 7.0 </code></pre></li> </ul> <p>Finally, note that <code>const</code> can also be used within a class or interface to define a <a href="http://php.net/manual/en/language.oop5.constants.php" rel="noreferrer">class constant</a> or interface constant. <code>define</code> cannot be used for this purpose:</p> <pre><code>class Foo { const BAR = 2; // valid } // but class Baz { define('QUX', 2); // invalid } </code></pre> <p><strong>Summary</strong></p> <p>Unless you need any type of conditional or expressional definition, use <code>const</code>s instead of <code>define()</code>s - simply for the sake of readability!</p>
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