PHP 5: const vs static

In the context of a class, static variables are on the class scope (not the object) scope, but unlike a const, their values can be changed.

class ClassName {
    static $my_var = 10;  /* defaults to public unless otherwise specified */
    const MY_CONST = 5;
}
echo ClassName::$my_var;   // returns 10
echo ClassName::MY_CONST;  // returns 5
ClassName::$my_var = 20;   // now equals 20
ClassName::MY_CONST = 20;  // error! won't work.

Public, protected, and private are irrelevant in terms of consts (which are always public); they are only useful for class variables, including static variable.

• public static variables can be accessed anywhere via ClassName::$variable.
• protected static variables can be accessed by the defining class or extending classes via ClassName::$variable.
• private static variables can be accessed only by the defining class via ClassName::$variable.

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Edit: It is important to note that PHP 7.1.0 introduced support for specifying the visibility of class constants.

One last point that should be made is that a const is always static and public. This means that you can access the const from within the class like so:

class MyClass
{
     const MYCONST = true;
     public function test()
     {
          echo self::MYCONST;
     }
}

From outside the class you would access it like this:

echo MyClass::MYCONST;