How to use Normal Variable inside Static Method

class Exam {
  public $foo = 1;
  public static function increaseFoo(){
    $this->foo++;
    echo $this->foo;
  }
}

Exam::increaseFoo();

This code generate an Error

E_ERROR : type 1 -- Using $this when not in object context -- at line 5

Is that possible to use global variable into static mathod?

Answer

replace $this with self, also you must mark your variable as static when using it in a static method:

class Exam {
  public static $foo = 1;
  public static function increaseFoo(){
    self::$foo++;
    echo self::$foo;
  }
}

Exam::increaseFoo();