PHP Constants Example

Last modified on: October 19, 2024

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<h3>PHP Constants</h3>
<p>In PHP, <strong>constants</strong> are defined values that cannot be changed once they are set. Constants are typically used to store values that remain the same throughout the execution of the script, such as configuration settings, mathematical values, or other static information. Unlike variables, constants do not begin with a dollar sign (<code>$</code>), and they can be accessed globally throughout the script.</p>
<hr />
<h3>Defining Constants</h3>
<p>PHP constants can be defined using two methods:</p>
<ol>
<li><strong><code>define()</code> function</strong> (case-sensitive by default)</li>
<li><strong><code>const</code> keyword</strong> (introduced in PHP 5.3 and case-sensitive)</li>
</ol>
<h4>1. Using <code>define()</code></h4>
<p>The <code>define()</code> function is used to declare a constant, with the first argument being the name of the constant and the second being its value.</p>
<pre class="language-php"><code>define("PI", 3.14159);
define("GREETING", "Welcome to PHP!");

echo PI;          // Output: 3.14159
echo GREETING;    // Output: Welcome to PHP!</code></pre>
<h4>2. Using <code>const</code></h4>
<p>The <code>const</code> keyword can be used to define constants in classes or globally, and it allows for better readability.</p>
<pre class="language-php"><code>const TAX_RATE = 0.15;
echo TAX_RATE;    // Output: 0.15</code></pre>
<hr />
<h3>Constant Naming Rules</h3>
<ul>
<li>Constants are usually written in <strong>uppercase</strong> letters (by convention).</li>
<li>A valid constant name starts with a letter or an underscore (<code>_</code>), followed by any number of letters, numbers, or underscores.</li>
<li>Constants are <strong>global</strong> by default and can be accessed from any scope in the script.</li>
</ul>
<hr />
<h3>Accessing Constants in a Class</h3>
<p>When constants are defined inside a class, they are accessed using the <strong>scope resolution operator</strong> (<code>::</code>).</p>
<pre class="language-php"><code>class Car {
    const WHEELS = 4;

public function getWheels() {
        return self::WHEELS; // Accessing class constant using 'self::'
    }
}

echo Car::WHEELS; // Output: 4

$car = new Car();
echo $car-&gt;getWheels(); // Output: 4</code></pre>
<p>In this example, the constant <code>WHEELS</code> is defined within the class <code>Car</code> and is accessed using <code>Car::WHEELS</code> outside the class or <code>self::WHEELS</code> within the class.</p>
<hr />
<h3>Case-Insensitive Constants</h3>
<p>By default, constants are <strong>case-sensitive</strong>. However, when using the <code>define()</code> function, you can make a constant case-insensitive by passing a third argument as <code>true</code>.</p>
<pre class="language-php"><code>define("HELLO", "Hello, World!", true);

echo hello; // Output: Hello, World!</code></pre>
<p>This feature is rarely used, as constants are usually defined in uppercase letters to maintain clarity and convention.</p>
<hr />
<h3>Differences Between Constants and Variables</h3>
<table>
<thead>
<tr>
<th>Aspect</th>
<th>Constants</th>
<th>Variables</th>
</tr>
</thead>
<tbody>
<tr>
<td>Declaration</td>
<td>Using <code>define()</code> or <code>const</code></td>
<td>Using <code>$</code> symbol</td>
</tr>
<tr>
<td>Modifiability</td>
<td>Cannot be changed once defined</td>
<td>Value can be changed</td>
</tr>
<tr>
<td>Scope</td>
<td>Global scope</td>
<td>Can have local, global, or static scope</td>
</tr>
<tr>
<td>Naming Convention</td>
<td>Typically uppercase letters</td>
<td>Typically camelCase or snake_case</td>
</tr>
<tr>
<td>Access inside class</td>
<td>Using <code>self::CONSTANT_NAME</code> or <code>Class::CONSTANT_NAME</code></td>
<td>Direct variable access using <code>$this-&gt;variable</code></td>
</tr>
</tbody>
</table>
<hr />
<h3>Magic Constants in PHP</h3>
<p>PHP also provides <strong>magic constants</strong>, which are predefined constants that change based on where they are used. These constants provide useful information about the current code execution, such as the file name, line number, and function name.</p>
<h4>Common Magic Constants:</h4>
<ol>
<li><strong><code>__LINE__</code></strong>: The current line number in the script.</li>
<li><strong><code>__FILE__</code></strong>: The full path and filename of the file.</li>
<li><strong><code>__DIR__</code></strong>: The directory of the file.</li>
<li><strong><code>__FUNCTION__</code></strong>: The function name.</li>
<li><strong><code>__CLASS__</code></strong>: The class name.</li>
<li><strong><code>__METHOD__</code></strong>: The class method name.</li>
<li><strong><code>__NAMESPACE__</code></strong>: The current namespace.</li>
</ol>
<p>Example:</p>
<pre class="language-php"><code>echo __LINE__;    // Output: Current line number
echo __FILE__;    // Output: Full path and filename of this file</code></pre>
<hr />
<h3>Practical Example of Constants</h3>
<p>Let&rsquo;s create a constant for a <strong>tax rate</strong> and use it to calculate the total cost after tax:</p>
<pre class="language-php"><code>define("TAX_RATE", 0.07); // Define a constant for tax rate

function calculateTotal($price) {
    return $price + ($price * TAX_RATE); // Using constant inside a function
}

echo calculateTotal(100); // Output: 107 (7% tax added)</code></pre>
<p>Here, the constant <code>TAX_RATE</code> is defined globally and is accessed within the <code>calculateTotal()</code> function to calculate the total price including tax.</p>
<hr />
<h3>&nbsp;</h3>
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