Por qué no

public static $CURRENT_TIME = time() + 7200;

work (Error):

Error de análisis: error de sintaxis, inesperado '('

per

class Database {
  public static $database_connection;

  private static $host = "xxx";
  private static $user = "xxx";
  private static $pass = "xxx";
  private static $db = "xxx";

  public static function DatabaseConnect(){
    self::$database_connection = new mysqli(self::$host,self::$user,self::$pass,self::$db);
    self::$database_connection->query("SET NAMES 'utf8'");
    return self::$database_connection;
  }
}

funciona

Soy nuevo en OOP, estoy muy confundido.