How does static public work for php?

<?php 

class Address {

    static public $valid_address_types = array(
        1 => 'Residence',
        2 => 'Business',
        3 => 'Park',
        );

    public $street_address_1;
    public $street_address_2;

    // Name of the city
    public $city_name;

    // name of the subdivision.
    public $subdivision_name;

    // Postal code
    protected $_postal_code;
    // name of the country

    public $country_name;

    // primary key of an address
    protected $_address_id;

    // when the record was created and last updated
    protected $_time_created;
    protected $_time_updated;
    // magic get; returns mixed

    function __construct($data = array()) {
        $this->_time_created = time();

        // ensure the property can be populated
        if (!is_array($data)) {
            trigger_error('Unable to construct address with a ' . get_class($name));
        }
        // if there is at least one value, populate the address with it.
        if (count($data) > 0) {
            foreach ($data as $name => $value) {
                // special case for protected properties
                if (in_array($name, array( 'time_created', 'time_updated'))) {
                    $name = '_' . $name;
                }
                $this->$name = $value;
            }
        }
    }

    function __get($name) {
        // Postal code lookup if unset.
        if (!$this->_postal_code) {
            $this->_postal_code = $this->_postal_code_guess();
        }
        // attempt to return a protected property by name.
        $protected_property_name = '_' . $name;
        if (property_exists($this, $protected_property_name)) {
            return $this->$protected_property_name;
        }
        trigger_error('Undefinited property via __get ' . $name);
        return NULL;
    }

    protected function _postal_code_guess() {
        return 'LOOKUP';
    }

    function __set($name, $value) {
        // Allow anything to set the postal code.
        if ('postal_code' == $name) {
            $this->$name = $value;
            return;
        }
        // unable access property trigger error
        trigger_error('Undefined or unallowed property via __set(): ' . $name);
    }

    // display an addres in html

    function display () {
        $output = '';

        // street address
        $output .= $this->street_address_1;
        if ($this->street_address_2) {
            $output .= '<br />' . $this->street_address_2;
        }

        // City, subdivision Postal.
        $output .= '<br/>' ;
        $output .= $this->city_name . ' , ' . $this->subdivision_name;
        $output .= ' ' . $this->postal_code;
        $output .= '<br/>';
        $output .= $this->country_name;
        return $output;
    }
    function __toString() {
        return $this->display();
    }
}

<?php 

require 'class.Address.inc';

?>

<h2>Instantiating address</h2><br>
<h2>Empty address</h2>
<?php 
$address = new Address;

echo '<tt><pre>' . var_export($address, TRUE) . '</pre></tt>';
 ?>
 <br>

 <?php 
$address->street_address_1 = '555 fake st';
$address->city_name = "Some City";
$address->subdivision_name = "State";
$address->postal_code = "90000";
$address->country_name = "USA";
echo '<tt><pre>' . var_export($address, TRUE) . '</pre></tt>';
  ?>
  <h2>Display Address...</h2>
  <?php 

echo $address->display();
   ?>
 <h3>testing magic __get and __set</h3>
 <?php 
 unset($address->postal_code);
 echo $address->display();

  ?>

  <h2>_construct with an array</h2>
  <?php 
  $address_2 = new Address(array(
    'street_address_1' => '123 place',
     'city_name' => 'Villageland',
     'subdivision_name' => 'Region',
     'postal_code' => '99999',
     'country_name' => 'canada',
    ));
  echo $address_2->display();

   ?>
   <h2>address __toString</h2>
   <?php 
   echo $address_2;

    ?>
<h2>Displaying Address Types..</h2>
<?php 

// why did it become :: this vs ->
echo "<tt><pre> " . var_export(Address::$valid_address_types, TRUE) . " </tt></pre>";
 ?>