Before I continue demonstrating how useful a single static property can be in the context of object-oriented programming with PHP 5, I want to reintroduce the hands-on example developed in the preceding article of this series. This will make it fresh in your mind, which should really help if you want to reproduce the business logic implemented by it within your own PHP applications.

As you probably recall, the example in question illustrated how a group of instances belonging to the same originating class can be easily linked with each other by using a single static property. The signatures corresponding each of the sample classes used for that specific occasion looked like this:

// define abstract 'User' class - declares '$nextUserID' property
static (it can be accessed by all the classes)
abstract class User{
   static public $nextUserID=1;
   public $userID;
   abstract public function __construct($fname,$lname,$email);
   abstract public function getFirstName();
   abstract public function getLastName();
   abstract public function getEmail();
   abstract public function getID();
}

// define 'ForumUser' class
class ForumUser extends User{
   public function __construct
($fname='John',$lname='Doe',$email='john@domain.com'){
      if(!$fname){
         throw new Exception('Invalid First Name for forum
user.');
      }
      if(!$lname){
         throw new Exception('Invalid Last Name for forum
user.');
      }
      if(!$email||preg_match("/^.+@.+..+$/",$email)){
         throw new Exception('Invalid email address for forum
user.');
      }
      $this->fname=$fname;
      $this->lname=$lname;
      $this->email=$email;
      $this->userID=self::$nextUserID++;
   }

   // get user's First Name
   public function getFirstName(){
      return $this->fname;
   }

   // get user's Last Name
   public function getLastName(){
      return $this->lname;
   }

   // get user's email address
   public function getEmail(){
      return $this->email;
   }

   // get user's ID
   public function getID(){
      return $this->userID;
   }
}
try{
   // create some forum user objects
   $user1=new ForumUser('Robert','Wilson','bob@domain.com');
   $user2=new ForumUser('John','Smith','johnny@domain.com');
   $user3=new ForumUser('Susan','Jackson','suse@domain.com');
   $user4=new ForumUser('Mary','King','mary@domain.com');

   // display information on first forum user
   echo '<h2>Data for first forum user is as following:</h2>';
   echo '<p>First Name: '.$user1->getFirstName().'</p><p>Last
Name: '.$user1->getLastName().'<p/><p>Email address: '.$user1-
>getEmail().'</p><p>User ID: '.$user1->getID().'</p>';
   /*
   displays the following:
   
Data for first forum user is as following:
   First Name: Robert
   Last Name: Wilson
   Email address: bob@domain.com
   User ID: 1
   */   

   // display information on second forum user
   echo '<h2>Data for second forum user is as following:</h2>';
   echo '<p>First Name: '.$user2->getFirstName().'</p><p>Last
Name: '.$user2->getLastName().'<p/><p>Email address: '.$user2-
>getEmail().'</p><p>User ID: '.$user2->getID().'</p>';
   /*
   displays the following:

Data for second forum user is as following:
   First Name: John
   Last Name: Smith
   Email address: johnny@domain.com
   User ID: 2
   */

   // display information on third forum user
   echo '<h2>Data for third forum user is as following:</h2>';
   echo '<p>First Name: '.$user3->getFirstName().'</p><p>Last
Name: '.$user3->getLastName().'<p/><p>Email address: '.$user3-
>getEmail().'</p><p>User ID: '.$user3->getID().'</p>';
   /*
   displays the following:

Data for third forum user is as following:
   First Name: Susan
   Last Name: Jackson
   Email address: suse@domain.com
   User ID: 3
   */

   // display information on last forum user
   echo '<h2>Data for last forum user is as following:</h2>';
   echo '<p>First Name: '.$user4->getFirstName().'</p><p>Last
Name: '.$user4->getLastName().'<p/><p>Email address: '.$user4-
>getEmail().'</p><p>User ID: '.$user4->getID().'</p>';
   /*
   displays the following:

Data for last forum user is as following:
   First Name: Mary
   Last Name: King
   Email address: mary@domain.com
   User ID: 4
   */
}
catch(Exception $e){
   echo $e->getMessage();
   exit();
}

As you can see, the previous sample classes implement a primitive application for displaying some basic data about a few fictional users, such as their first and last names, along with their email addresses. However, the most interesting thing to note here is the utilization of a single static "$nextUserID" property, which is used by all the instances of the originating "ForumUser" class to increment the ID of the next user. Indeed, this implementation of a given static property shows in a nutshell its remarkable functionality, particularly when it comes to enchaining several instances of a specific class.

So far, so good. At this stage, you hopefully grasped the logic that drives the above hands-on example. So let's take a look at another case where the use of a single static property can be really handy.

In this case, I'll show you how to build an expandable data validation system utilizing a basic static property in conjunction with the programmatic model dictated by the popular Observer design pattern.

Want to see how this will be achieved? Jump ahead and read the next few lines.