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 10echo ClassName::MY_CONST;  // returns 5ClassName::$my_var = 20;   // now equals 20ClassName::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.

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;

Constant is just a constant, i.e. you can't change its value after declaring.

Static variable is accessible without making an instance of a class and therefore shared between all the instances of a class.

Also, there can be a static local variable in a function that is declared only once (on the first execution of a function) and can store its value between function calls, example:

function foo(){   static $numOfCalls = 0;   $numOfCalls++;   print("this function has been executed " . $numOfCalls . " times");}