Protected Visibility

The protected keyword is part of the three keywords to define visibility of a method, property or constant. It is the strictest level of visibility : it restricts usage to the current class and its parents and children.

<?php

class x {
    protected const X = 1;

    public function method() {
        echo self::X;
    }
}

?>

Documentation

Related : Final Keyword, Visibility, Private Visibility, Public Visibility, Var