difference between _postal_code postal_code

<?php

require 'class.Address.inc';

echo '<h2>Instantiating Address</h2>';
$address = new Address;

echo '<h2>Empty Address</h2>';
echo '<tt><pre>' . var_export($address, TRUE) . '</pre></tt>';

echo '<h2>Setting properties...</h2>';
$address->street_address_1 = '555 Fake Street';
$address->city_name = 'Townsville';
$address->subdivision_name = 'State';
$address->postal_code = '12345';
$address->country_name = 'United States of America';
echo '<tt><pre>' . var_export($address, TRUE) . '</pre></tt>';

echo '<h2>Displaying address...</h2>';
echo $address->display();

echo '<h2>Testing protected access.</h2>';
echo "<br>Address ID: {$address->_address_id}";
echo "<br>time created: {$address->_time_created}";

echo '<h2>Testing magic __get and __set</h2>';
unset($address->postal_code);
unset($address->_postal_code);
echo $address->display();

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<?php

/**
 * Physical address. 
 */
class Address {
  // Street address.
  public $street_address_1;
  public $street_address_2;
  
  // Name of the City.
  public $city_name;
  
  // Name of the subdivison.
  public $subdivision_name;
  
  // Postal code.
  protected $_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
  * @param string $name
  * @return mixed
  */
  
  function __get($name){
    echo '<h1>'.__FUNCTION__.' '.$name.'</h1>';
    //postal code lookup if unset
   // if (!$this->_postal_code){
      $this->_postal_code = '_postalcode';
      $this->postal_code = 'postalcode';
      //$this->_postal_code = $this->_postal_code_guess();
    //}
    
  //attempt to return a protected property by name.
  //$protected_property_name = '_' . $name;
  
  if ($name=='_postal_code' || $name=='postal_code'){
    $protected_property_name=$name;
  }
  
  if (property_exists($this, $protected_property_name)){
    return $this->protected_property_name;
  }
  
  //unable to access a property; trigger error.
  trigger_error('Undefined property via __get: ' . $name);
  return NULL;
    
  }
  /**
  * magic __set
  * 
  * @param string $name
  * @param mixed $value
  */
   function __set($name,$value){
     //Allow anything to set the postal code
     if ('postal_code' == $name || '_postal_code' == $name){
       $this->$name = $value;
       return;
     }
     
     //unable to access property; trigger error.
     trigger_error('Undefined or unallowed property via __set(): '.$name);
   }
  
  
  /**
  * guess the postal code given the subdivision and city name
  * @todo replace with a database lookup
  * @return string
  * 
  */
  protected function _postal_code_guess(){
    return 'LOOKUP';
  }
  
  /**
   * Display an address in HTML.
   * @return string 
   */
  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 .= ' _pc:' . $this->_postal_code. ' pc:'.$this->postal_code;
    
    // Country.
    $output .= '<br/>';
    $output .= $this->country_name;
    
    return $output;
  }
}

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Instantiating Address
Empty Address
Address::__set_state(array(
   'street_address_1' => NULL,
   'street_address_2' => NULL,
   'city_name' => NULL,
   'subdivision_name' => NULL,
   '_postal_code' => NULL,
   'postal_code' => NULL,
   'country_name' => NULL,
   '_address_id' => NULL,
   '_time_created' => NULL,
   '_time_updated' => NULL,
))
Setting properties...
Address::__set_state(array(
   'street_address_1' => '555 Fake Street',
   'street_address_2' => NULL,
   'city_name' => 'Townsville',
   'subdivision_name' => 'State',
   '_postal_code' => NULL,
   'postal_code' => '12345',
   'country_name' => 'United States of America',
   '_address_id' => NULL,
   '_time_created' => NULL,
   '_time_updated' => NULL,
))
Displaying address...
555 Fake Street
Townsville, State _pc: pc:12345
United States of America
Testing protected access.
__get _address_id

Notice: Undefined property via __get: _address_id in C:\wamp\www\Ex_Files_oophp-edit2\03_02\class.Address.inc on line 58

Address ID: 
__get _time_created

Notice: Undefined property via __get: _time_created in C:\wamp\www\Ex_Files_oophp-edit2\03_02\class.Address.inc on line 58

time created: 
Testing magic __get and __set

Fatal error: Cannot access protected property Address::$postal_code in C:\wamp\www\Ex_Files_oophp-edit2\03_02\demo.php on line 27

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