As I explained in the introduction, my intention here is to develop a simple abstraction layer by using only a bridge class and a couple of "bridged" objects. The purpose of doing this is basically to have access to the MySQL server by utilizing either the conventional "mysql" extension, or the highly-improved "mysqli" library that comes bundled with PHP 5.

Having explained how I plan to use the bridge design pattern in this specific case, let me show you the first class involved in this example. It consists of a simple wrapper for MySQL, and logically uses the respective "mysql" extension as well. Here is the signature for this brand new class. Please have a look at it: 

// define 'MySQL' class

class MySQL{

	public function __construct($host,$user,$password,$database){

		if(!$conId=mysql_connect($host,$user,$password)){

			throw new Exception('Error connecting to 

the server');
		}
	       
		if(!mysql_select_db($database,$conId)){

			throw new Exception('Error selecting 

database');
		}

	}

// run query

public function query($query){

	if(!$this->result=mysql_query($query)){

		throw new Exception('Error performing query '.$query);
        }
	
	return new Result($this,$this->result);

	}

}

// define 'Result' class

class Result {

	private $mysql;

	private $result;

	public function __construct(&$mysql,$result){

       		$this->mysql=&$mysql;
       
		$this->result=$result;
	}

	// fetch row

	public function fetch_array(){

        	return mysql_fetch_assoc($this->result);

	}

	// count rows

	public function countRows(){
    
		if(!$rows=mysql_num_rows($this->result)){

			throw new Exception('Error counting rows');

		}

       		return $rows;
	}

	// count affected rows

	public function countAffectedRows(){

		if(!$rows=mysql_affected_rows($this->mysql->conId)){

			throw new Exception('Error counting affected 

rows');
		}

		return $rows;
	}

	// get ID form last-inserted row

	public function getInsertID(){
       
		if(!$id=mysql_insert_id($this->mysql->conId)){

			throw new Exception('Error getting ID');

		}

		return $id;
	}

	// seek row

	public function seekRow($row=0){
       
		if(!is_int($row)||$row<0){

			throw new Exception('Invalid result set 

offset');
	        }
       
		if(!mysql_data_seek($this->result,$row)){

			throw new Exception('Error seeking data');
	        }
	}

} 

As you can see, there's nothing special with reference to the MySQL wrapping classes listed above. Similar to many abstraction classes that you have possibly used in many web applications, these allow you to perform some simple operations, such as connecting to the server, selecting a specific database, running queries, and processing result sets in a few useful ways.

However, the pair of MySQL wrapping classes that you just saw definitely aren't the main topic of this article. I defined them here only with the purpose of creating another application that lets you access the MySQL server, either with the traditional "mysql" extension, or the improved "mysqli" library.

In this case, the first requirement has been already satisfied, since the previous classes use the conventional extension. Therefore, it's time to move on and see how to develop a bridge class which will allow us to use MySQL by utilizing the two mentioned libraries.

Things are getting really exciting now! Therefore, if you want to learn how this bridge class will be created, please keep reading.