<?php

class TownCenter {

    public static $hit_points = 1800;
    public static $build_time = 10080;
    public static $build_age = 3;
    public static $deffence = 0;
    public static $ability = array();
    // here ->
    public static $population = array( 1 => '5', 2 => '10', 3 => '15', 4 => '20', 5 => '20' );

    public static function ability() {
         var_dump(TownCenter::$population);
    }

    protected function backingOfScience() {

    }

}

$f = new TownCenter();

var_dump($f);
TownCenter::ability();

?>