Using OOP in PHP: Practical Example

Object Oriented design is particularly useful where you have data objects that relate to one another – so it is very common for example to have OOP used to deal with data. PHP 5 has the PDO (PHP Data Object) classes which are a great way to talk to a backend database, either mysql or any other kind of database, and using the object oriented interface offered by this extension ties in well with the usage I have shown here.

A common mistake for those coming to OO for the first time is to declare methods inside a class but then instantiate a single copy of that and pass in data such as the ID of the data to each method. Here’s an example of a user class that does this:

class User {
    /**
     * getDisplayName
     *
     * @param int $user_id the user_id of the user in the database table
     * @access public
     * @return string Display name of this user
     */
    public function getDisplayName($user_id) {
        $sql = 'select display_name from users where user_id = '.$user_id;
        $results = mysql_query($sql);
        $row = mysql_fetch_array($results);
        return $row['display_name'];
    }

    /**
     * getFriends
     *
     * @param int $user_id the user_id of the user in the database table
     * @access public
     * @return array An array of friend_ids
     */
    public function getFriends($user_id) {
        $sql = 'select friend_id from user_friends where user_id = '.$user_id;
        $results = mysql_query($sql);
        $friends = array();
        while($row = mysql_fetch_array($results)) {
            $friends[] = $row['friend_id'];
        }
        return $friends;
    }
}

user.php

And some code that uses it:

include('user.php');
mysql_connect('localhost','root','');
mysql_select_db('test');

$user = new User();
echo "User known as: " . $user->getDisplayName(1) . "\n";

$friends = $user->getFriends(1);
echo "Friends with: " . implode(', ',$friends) . "\n";

If you want to run this code yourself, then you can set up the database tables using this script:

CREATE TABLE `user_friends` (
  `user_friend_id` int(11) NOT NULL AUTO_INCREMENT,
  `user_id` int(11) NOT NULL,
  `friend_id` int(11) NOT NULL,
  PRIMARY KEY (`user_friend_id`)
)
INSERT INTO `user_friends` VALUES (1,1,3),(2,1,5);

CREATE TABLE `users` (
  `user_id` int(11) NOT NULL AUTO_INCREMENT,
  `first_name` varchar(20) DEFAULT NULL,
  `last_name` varchar(50) DEFAULT NULL,
  `display_name` varchar(50) DEFAULT NULL,
  PRIMARY KEY (`user_id`)
)
INSERT INTO `users` VALUES (1,'Lorna','Mitchell','lornajane');

user.sql

This is more like a function library than an actual object, and although it works perfectly well and does have its applications, it can be better to aim to take advantage of more object oriented features. To do this, a new object should be created for each item the system handles, and if it has an ID and perhaps some other properties, these should be set as part of the object. Then rather than creating a single user object and doing getFriends(42) on it, you’d have a user with ID 42 which did $user->getFriends().

Here’s a follow up example with a class and how to use it, this will give better performance and enable you to clearly see which data goes with which object where there are multiple items on a page:

class User {
    /**
     * getUserById
     *
     * @param int $user_id the id of the user row in the table
     * @access public
     * @return true if the user was found
     */
    public function getUserById($user_id) {
        $sql = 'select first_name, last_name, display_name from users where user_id = ' . $user_id;
        $results = mysql_query($sql);
        $row = mysql_fetch_array($results);
        $this->user_id = $user_id;
        $this->first_name = $row['first_name'];
        $this->last_name = $row['last_name'];
        $this->display_name = $row['display_name'];

        // in real life, there would be escaping and error handling and we'd only return true if we got data
        return true;
    }

    /**
     * getDisplayName
     *
     * @access public
     * @return string Display name of this user
     */
    public function getDisplayName() {
        return $this->display_name;
    }

    /**
     * getFriends
     *
     * @access public
     * @return array An array of friend_ids
     */
    public function getFriends() {
        $sql = 'select friend_id from user_friends where user_id = '.$this->user_id;
        $results = mysql_query($sql);
        $friends = array();
        while($row = mysql_fetch_array($results)) {
            $friends[] = $row['friend_id'];
        }
        return $friends;
    }
}

user2.php

include('user2.php');
mysql_connect('localhost','root','');
mysql_select_db('test');

$user = new User();
// populate the object
$user->getUserById(1);
echo "User known as: " . $user->getDisplayName() . "\n";

$friends = $user->getFriends();
echo "Friends with: " . implode(', ',$friends) . "\n";

In Summary

This has been a very introductory look at some of the object oriented features available in PHP, to get you started out with the basics of working with OO code and also writing your own. If this has helped you take your first OOP steps, then let us know by leaving a comment and let us know what you built! In the next post there will be some more in-depth content including using constructors, access modifiers and working with the static keyword in PHP – taking your skills to the next level.