{"id":1735,"date":"2021-05-18T14:25:08","date_gmt":"2021-05-18T14:25:08","guid":{"rendered":"https:\/\/phptutorial.net\/?page_id=1735"},"modified":"2025-04-07T11:34:35","modified_gmt":"2025-04-07T11:34:35","slug":"php-class-constants","status":"publish","type":"page","link":"https:\/\/www.phptutorial.net\/php-oop\/php-class-constants\/","title":{"rendered":"PHP Class Constants"},"content":{"rendered":"\n

Summary<\/strong>: in this tutorial, you will learn about PHP class constants and how to use them effectively.<\/p>\n\n\n\n

Introduction to the PHP class constants #<\/a><\/h2>\n\n\n\n

Sometimes, you need to define a constant<\/a> that is specific to a class<\/a>. In this case, you can use the PHP class constants.<\/p>\n\n\n\n

To define a constant of a class, you use the const<\/code> keyword. For example:<\/p>\n\n\n

<?php<\/span>\n\nclass<\/span> Circle<\/span>\n<\/span>{\n    const<\/span> PI = M_PI;\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we define the PI<\/code> constant in the Circle<\/code> class. By convention, a constant name is in uppercase. If the constant name contains multiple words, you can use the underscore (_<\/code>) to separate the words, for example MY_CONSTANT<\/code>.<\/p>\n\n\n\n
Since a constant is defined per class, not per instance of the class, you use the self<\/code> keyword to reference the constant inside the class. For example:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> Circle<\/span>\n<\/span>{\n    const<\/span> PI = M_PI;\n\n    private<\/span> $radius;\n\n    public<\/span> function<\/span> __construct<\/span>(float $radius)<\/span>\n    <\/span>{\n        $this<\/span>->radius = $radius;\n    }\n\n    public<\/span> function<\/span> area<\/span>()<\/span>: float<\/span>\n    <\/span>{\n        return<\/span> self<\/span>::PI * $this<\/span>->radius ** 2<\/span>;\n    }\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, we define the Circle<\/code> class with the $radius<\/code> property. Inside the area()<\/code> method, we calculate the area of the circle using the radius and the self::PI<\/code> constant.<\/p>\n\n\n\n
When you define a constant in a class, its visibility is public by default. It means that you can also access the constant outside of the class.<\/p>\n\n\n\n
To reference to the constant outside the class, you use the class name and ::<\/code> operator like this:<\/p>\n\n\n
<?php<\/span>\n\n\/\/  Circle class<\/span>\n\/\/  ...<\/span>\n\necho<\/span> Circle::PI;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
Note that you can reference the class using a variable with the value is the class name. For example:<\/p>\n\n\n
<?php<\/span>\n\n$className = 'Circle'<\/span>;\n\necho<\/span> $className::PI;<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, instead of using Circle::PI<\/code>, we use the $className::PI<\/code> to reference the PI constant.<\/p>\n\n\n\n
Since PHP 7.1.0, you can use visibility modifier keywords such as private<\/code>, protected<\/code>, and public<\/code> with the class constant. For example:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> Circle<\/span>\n<\/span>{\n    private<\/span> const<\/span> PI = M_PI;\n\n    \/\/ other methods   <\/span>\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
In this example, the PI<\/code> constant is private and cannot be used outside the Circle<\/code> class.<\/p>\n\n\n\n
It’s possible to define a class constant using a constant expression. A constant expression is an expression that contains only constants. For example:<\/p>\n\n\n
<?php<\/span>\n\nclass<\/span> MyClass<\/span> \n<\/span>{\n    const<\/span> ONE_THIRD = 1<\/span>\/3<\/span>;\n}<\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
PHP class constants and inheritance #<\/a><\/h2>\n\n\n\n
The following example illustrates how to define a constant in the parent class and override it in the child class. For example:<\/p>\n\n\n
<?php<\/span>\n\nabstract<\/span> class<\/span> Model<\/span>\n<\/span>{\n    protected<\/span> const<\/span> TABLE_NAME = ''<\/span>;\n\n    public<\/span> static<\/span> function<\/span> all<\/span>()<\/span>\n    <\/span>{\n        return<\/span> 'SELECT * FROM '<\/span> . static<\/span>::TABLE_NAME;\n    }\n}\n\nclass<\/span> User<\/span> extends<\/span> Model<\/span>\n<\/span>{\n    protected<\/span> const<\/span> TABLE_NAME = 'users'<\/span>;\n}\n\nclass<\/span> Role<\/span> extends<\/span> Model<\/span>\n<\/span>{\n    protected<\/span> const<\/span> TABLE_NAME = 'roles'<\/span>;\n}\n\necho<\/span> User::all() . '<br>'<\/span>; \/\/ SELECT * FROM users;<\/span>\necho<\/span> Role::all() . '<br>'<\/span>; \/\/ SELECT * FROM roles;<\/span><\/code><\/span>Code language:<\/span> PHP<\/span> (<\/span>php<\/span>)<\/span><\/small><\/pre>\n\n\n
How it works.<\/p>\n\n\n\n
First, define a constant TABLE_NAME<\/code> in the Model<\/code> class. In the all()<\/code> static method, returns a query that selects all rows from the table name specified by the TABLE_NAME<\/code> constant.<\/p>\n\n\n\n
Second, define the User<\/code> and Role<\/code> classes that extend the Model<\/code> class. In the User and Role class, redefine the TABLE_NAME<\/code> constant.<\/p>\n\n\n\n
Since the User<\/code> and Role<\/code> classes inherits the all()<\/code> method of the Model<\/code> class, they can call the all()<\/code> method.<\/p>\n\n\n\n
When the User<\/code> class calls the all()<\/code> method, the all()<\/code> method returns the expected TALBE_NAME<\/code> constant defined in the User<\/code> class. The same logic is applied to the Role<\/code> class.<\/p>\n\n\n\n
Summary #<\/a><\/h2>\n\n\n\n
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  • Use the const<\/code> keyword to define a class constant.<\/li>\n\n\n\n
  • Use the visibility modifier keywords, including public, protected, and private. By default, a class constant is public.<\/li>\n<\/ul>\n
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Try it<\/a><\/p>\n\n\n\n
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