Detailed explanation of php class constant definition and examples

What are class constants?

In php, we can understand that a quantity whose value does not change is called a constant. So, what is a class constant? In fact, class constants are also easy to understand. We can call the value that always remains unchanged in the class a constant, and this constant can be called a class constant. Be sure to remember that you do not need to use the "$" symbol when defining and using constants.

Class constants belong to the class itself, not to object instances, and cannot be accessed through object instances

* Cannot be modified with public, protected, private, static

* Subclasses can be repeated To write constants in the parent class, you can call the constants in the parent class through (parent::)

* Since PHP5.3.0, you can use a variable to dynamically call the class. But the value of this variable cannot be a keyword (such as self, parent or static).

Definition of class constants

Class constants are defined using the const keyword:

const 常量名 = 常量值

Example

Define and use a class constant

<?php
header("content-type:text/html;charset=utf-8");
/**
 * PHP类常量
 *
 * 类常量属于类自身，不属于对象实例，不能通过对象实例访问
 * 不能用public，protected，private，static修饰
 * 子类可以重写父类中的常量，可以通过（parent::)来调用父类中的常量
 * 自PHP5.3.0起，可以用一个变量来动态调用类。但该变量的值不能为关键字（如self，parent或static）。
 */
class Foo
{
    // 常量值只能是标量，string，bool，integer，float，null，可以用nowdoc结构来初始化常量
    const BAR = &#39;PHP中文网&#39;;
    public static function getConstantValue()
    {
        // 在类的内部可以用self或类名来访问自身的常量，外部需要用类名
        return self::BAR;
    }
    public function getConstant()
    {
        return self::BAR;
    }
}
$foo = &#39;Foo&#39;;
echo $foo::BAR, &#39;<br />&#39;;
echo Foo::BAR, &#39;<br />&#39;;
$obj = new Foo();
echo $obj->getConstant(), &#39;<br />&#39;;
echo $obj->getConstantValue(), &#39;<br />&#39;;
echo Foo::getConstantValue();
?>

Code running result:

Result In the above example, we use the subclass to rewrite the parent class constant, the code is as follows :

<?php
header("content-type:text/html;charset=utf-8");
/**
 * PHP类常量
 *
 * 类常量属于类自身，不属于对象实例，不能通过对象实例访问
 * 不能用public，protected，private，static修饰
 * 子类可以重写父类中的常量，可以通过（parent::)来调用父类中的常量
 * 自PHP5.3.0起，可以用一个变量来动态调用类。但该变量的值不能为关键字（如self，parent或static）。
 */
class Foo
{
    // 常量值只能是标量，string，bool，integer，float，null，可以用nowdoc结构来初始化常量
    const BAR = &#39;PHP中文网&#39;;
    public static function getConstantValue()
    {
        // 在类的内部可以用self或类名来访问自身的常量，外部需要用类名
        return self::BAR;
    }
    public function getConstant()
    {
        return self::BAR;
    }
}
$foo = &#39;Foo&#39;;
echo $foo::BAR, &#39;<br />&#39;;
echo Foo::BAR, &#39;<br />&#39;;
$obj = new Foo();
echo $obj->getConstant(), &#39;<br />&#39;;
echo $obj->getConstantValue(), &#39;<br />&#39;;
echo Foo::getConstantValue();
// 以上均输出PHP中文网
echo "<hr/>";
class Bar extends Foo
{
    const BAR = &#39;foo&#39;; // 重写父类常量
    public static function getMyConstant()
    {
        return self::BAR;
    }
    public static function getParentConstant()
    {
        return parent::BAR;
    }
}
echo Bar::getMyConstant(),&#39;<br/>&#39;; // foo
echo Bar::getParentConstant(); // PHP中文网
?>

Code running results:

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Detailed explanation of the definition and usage examples of PHP constants

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