Autoloading Explained

You can read about the PHP autoloading on the php website. Using the autoloader allows us to lazily load classes as they are requested instead of pre-loading all classes. This is a huge benefit to performance.

The way the Doctrine autoloader works is simple. Because our class names and paths are related, we can determine the path to a Doctrine class based on its name.

Imagine we have a class named Doctrine_Some_Class and we instantiate an instance of it:

$class = new Doctrine_Some_Class();

The above code will trigger a call to the Doctrine_Core::autoload() function and pass it the name of the class instantiated. The class name string is manipulated and transformed in to a path and required. Below is some pseudo code that shows how the class is found and required:

class Doctrine
{
    public function autoload($className)
    {
        $classPath = str_replace('_', '/', $className) . '.php';
        $path = '/path/to/doctrine/' . $classPath;
        require_once($path);
        return true;
    }
}

In the above example the Doctrine_Some_Class can be found at /path/to/doctrine/Doctrine/Some/Class.php.

Obviously the real Doctrine_Core::autoload() function is a bit more complex and has some error checking to ensure the file exists but the above code demonstrates how it works.