<?php
    /**
     * Define MyClass
     */
    class MyClass
    {
        // Declare a public constant
        public const MY_PUBLIC = 'public';

        // declare a protected constant
        protected const MY_PROTECTED = 'protected';

        // declare a private constant
        private const MY_PRIVATE = 'private';

        function foo()
        {
            echo self::MY_PUBLIC;
            echo self::MY_PROTECTED;
            echo self::MY_PRIVATE;
        }
    }

    $myclass = new MyClass();
    MyClass::MY_PUBLIC; // works
    MyClass::MY_PROTECTED; // fatal error
    MyClass::MY_PRIVATE; // fatal error
    $myclass->foo(); // public, protected and private work

    /**
     * define MyClass2
     */
    class MyClass2 extends MyClass
    {
        // this is public
        function foo2()
        {
            echo self::MY_PUBLIC;
            echo self::MY_PROTECTED;
            echo self::MY_PRIVATE; // fatal error
        }
    }

    $myclass2 = new MyClass2;
    echo MyClass2::MY_PUBLIC; // works
    $myclass2->foo2(); // public and protected work, not private
?>