performance of loading php classes and the use of 'extends'

Question

I have an includes.php page that I load at the start of every page of my website. As I develop the website, the number of classes that I am using is growing. So I end up with something like this:

$db = new DB($config);
$login = new Login($db, $config);
$form = new Form($db, $config);

And the list goes on and on. I have two questions about this practice:

First, Considering I might not be using a class at a certain page (I might not have a $form on every single page), how much does it really matter, performance-wise, to load this class every time any given page is loaded?

Second, you may have noticed that I am passing the class instance $db to all the other classes, as well as a variable $config. In the php code of every class, I do something like this:

public $db;
public $config;

public function __construct($db, $config, $smarty){
    $this->db = $db;
    $this->config = $config;
}

then in the class methods, I address the database and config files with 'this' as such:

public function myfunction(){
  $this->db;
  $this->config;
}

When should I use 'extends' rather than passing $db to the class, assuming every class uses the db? Does passing $db to every class hurt in any way in terms of performance?

Thanks!

Answer 1

When should I use 'extends' rather than passing $db to the class, assuming every class uses the db?

When it makes sense -- and only when it does !

You have at least two things to consider :

• "class A extends B" kind of means "class A **is a** B"
  • more clearly, a Car is a MotorVehicule ; a MotorVehicule is a Vehicule ; a Bus is a MotorVehicule ; a Bicycle is a Vehicule
  • however, a Ball is not a Vehicule
  • In your case, a Form is definitly not a DataBase ! Nor is a Login
• In PHP, a class can only extend one class
  • You can not have something being both a Vehicule and an Animal
  • But a Car is a MotorVehicule, which, itself, is a Vehicule :-)

In the case of a Database object (in your case, it's more a connection to a DB), mosts of your classes will not themselves "be" a database connection. So, they shouldn't extend that class.

However, they are using a DB connection (a Form "has a" DB connection) ; so, they should have a property representing that DB connection. That's what you are doing.

Instead of passing $db to each constructor, you might use

• either the Singleton design pattern
• or the Registry design pattern
• or some kind of global variable, but that's almost the same... just being worse (not OOP and all that) !

But passing the $db object is great for unit-testing, mock objects, and all that...
I think it could be considered as being the Dependancy Injection design pattern, btw (not sure, but looks like it)

---

About loading lots of classes, other people gave answers :

• Use autoloading if you can
• Use an opcode cache, like APC, if you can

Both of those are great suggestions that you should take into consideration ;-)

---

One last thing :

Does passing $db to every class hurt in any way in terms of performance?

Maybe it does a little bit ; but, honnestly, except if you are google and have millions of users... who cares ?

If you are doing a couple of DB queries, those will take LOTS of time, comparing to passing one more parameter to even a dozen methods !
So, the small amount of time used passing paremeters can probably be neglected :-)

Answer 2

Have you tried something like this?

function __autoload($class_name) {
 require_once("includes/php/class." . $class_name . ".php");
 }

So it only loads the class name when the class name is encountered. (Change the path to suit your php classes... mine are like class.Object.php, with the class name "Object").