Zend PHP 7 Certification – OOP – Class Constants

11th September 2017

This post covers the Class Constants section of the OOP chapter when studying for the Zend PHP 7 Certification.

Class constants are constant values on a per-class basis that remain the same and are unchangeable.

Constants differ from normal variables in that you don’t use the $ symbol to declare or use them.

They are also publicly visible and cannot be made protected or private. A simple class constant declaration looks like the following:

<?php
class MyClass
{
    const MYCONST = 'Some constant';
}

$class = new MyClass();
echo $class::MYCONST; // Outputs: Some constant

Constants are usually defined in uppercase and similar to variables they cannot start with a number. PHP 5.5 does not allow you to define an array constants. This is supported as of PHP 5.6, as PHP have made it possible to provide a scalar expression involving numeric and string literals.

<?php
class MyClass
{
    const MYCONST = array("apples", "bananas");
    const SEC_PER_DAY = 60 * 60 * 24;
}

$class = new MyClass();
print_r($class::MYCONST);

// PHP 5.5 output: Fatal error
// PHP 5.6+ output: Array ( [0] => apples [1] => bananas )

You can use the self keyword when referencing a constant within a class method.

class MyClass
{
    const CONSTANT = 'constant value';

    function showConstant() {
        echo self::CONSTANT . "\n";
    }
}

$class = new MyClass();
$class->showConstant(); // Outputs: constant value

As of PHP 5.3, you can use HEREDOC and NOWDOC syntax to assist defining constants.

<?php
class foo {
    const BAR = <<<'EOT'
bar
EOT;
    const BAZ = <<<EOT
baz
EOT;
}

$class = new foo();
echo $class::BAR . "\n";
echo $class::BAZ;

// Outputs:
bar
baz

As of PHP 7.1.0 visibility modifiers are allowed for class constants.

<?php
class Foo {
    public const BAR = 'bar';
    private const BAZ = 'baz';
}
echo Foo::BAR, PHP_EOL;
echo Foo::BAZ, PHP_EOL;

// Outputs:
bar

Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …

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Note: This article is based on PHP version 7.1.0.