PHP OOP - constant vs static variables?

Question

In PHP, What is is the difference between:

1. Constants and static variables?
2. Extending a class and creating its object?

I know how they can be used, but I can't clearly distinguish between them.

Answer 1

Comparison

Static

1. Can has a access modifier.

   class A{
       public static $public_static = "can access from anywhere";
       protected static $protected_static = "can access from inheriting classes";
       private static $private_static = "can access only inside the class";
   }
   

2. Depending the visibility you can access static variables.

   //inside the class
       self::$variable_name;
       static::$variable_name;
   //outside the class
       class_name::$variable_name;
   

3. Can change the value after declaration.

       self::$variable_name = "New Value";
       static::$variable_name = "New Value";
   

4. No need to initialize when declare.

       public static $variable_name; 
   

5. Normal variable declaration rules applied(ex: begins with $)

6. Can create inside a function.

       class A{
           function my_function(){
                static $val = 12;
                echo ++$val; //13
           }
       }
   

---

Constant

1. Always public cannot put access modifiers(>PHP7.1)

   class A{
       const my_constant = "constant value";
       public const wrong_constant="wrong" // produce a parse error
   }
   

2. Anywhere you can access constant.

   //inside the class
       self::variable_name;
       static::variable_name;
   //outside the class
       class_name::variable_name;
   

3. Cannot change the value after declaration.

   self::variable_name = "cannot change"; //produce a parse error
   

4. Must initialize when declare.

   class A{
       const my_constant = "constant value";// Fine
       const wrong_constant;// produce a parse error
   }
   

5. Must not use $ in the beginning of the variable(Other variable rules applied).

    class A{
       const my_constant = "constant value";// Fine
       const $wrong_constant="wrong";// produce a parse error
   }
   

6. Cannot declare inside a function.

---

When Extending

    class A{

        public static $public_static = "can access from anywhere";
        protected static $protected_static = "can access from inheriting classes";
        private static $private_static = "can access only inside the class";
        
        const my_constant = "Constant value";
    }

    class B extends A{

        function output(){

            // you can use self or static

            echo self::$public_static; //can access from anywhere;
            echo self::$protected_static; //can access from inheriting classes;
            self::$protected_static = "Changed value from Class B";
            echo self::$protected_static; //"Changed value from Class B";
            
            echo self::$private_static; //Produce Fatal Error

            echo self::my_constant;//Constant value
        }
    }