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copied!<blockquote> <p><em>When should I use 'extends' rather than passing $db to the class, assuming every class uses the db?</em></p> </blockquote> <p>When it makes sense -- and only when it does !</p> <p>You have at least two things to consider :</p> <ul> <li>"<code>class A extends B</code>" kind of means "<code>class A **is a** B</code>" <ul> <li>more clearly, a <code>Car</code> is a <code>MotorVehicule</code> ; a <code>MotorVehicule</code> is a <code>Vehicule</code> ; a <code>Bus</code> is a <code>MotorVehicule</code> ; a Bicycle is a <code>Vehicule</code></li> <li>however, a <code>Ball</code> is not a <code>Vehicule</code></li> <li>In your case, a <code>Form</code> is definitly not a <code>DataBase</code> ! Nor is a <code>Login</code></li> </ul></li> <li>In PHP, a class can only <code>extend</code> <strong>one</strong> class <ul> <li>You can not have something being both a <code>Vehicule</code> and an <code>Animal</code></li> <li>But a <code>Car</code> <strong>is a</strong> <code>MotorVehicule</code>, which, itself, <strong>is a</strong> <code>Vehicule</code> <strong>:-)</strong></li> </ul></li> </ul> <p>In the case of a Database object <em>(in your case, it's more a connection to a DB)</em>, mosts of your classes will not themselves "be" a database connection. So, they shouldn't extend that class.</p> <p>However, they are using a DB connection <em>(a <code>Form</code> "<strong>has a</strong>" DB connection)</em> ; so, they should have a property representing that DB connection. That's what you are doing.</p> <p><br> Instead of passing <code>$db</code> to each constructor, you might use</p> <ul> <li>either the <a href="http://en.wikipedia.org/wiki/Singleton" rel="noreferrer">Singleton</a> design pattern</li> <li>or the Registry design pattern</li> <li>or some kind of global variable, but that's almost the same... just being worse <em>(not OOP and all that)</em> !</li> </ul> <p>But passing the <code>$db</code> object is great for unit-testing, mock objects, and all that... <br>I think it could be considered as being the Dependancy Injection design pattern, btw <em>(not sure, but looks like it)</em></p> <hr> <p>About loading lots of classes, other people gave answers :</p> <ul> <li>Use autoloading if you can</li> <li>Use an opcode cache, like APC, if you can</li> </ul> <p>Both of those are great suggestions that you should take into consideration <strong>;-)</strong></p> <hr> <p>One last thing :</p> <blockquote> <p><em>Does passing $db to every class hurt in any way in terms of performance?</em></p> </blockquote> <p>Maybe it does a <strong>little</strong> bit ; but, honnestly, except if you are google and have millions of users... who cares ?</p> <p>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 ! <br>So, the small amount of time used passing paremeters can probably be neglected <strong>:-)</strong></p>

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