First, let's assume the following directory structure and class files, and that the top level directory and library directory are on the include_path:

Now, let's create a plugin loader to address the various view helper repositories available:

We can then load a given view helper using just the portion of the class name following the prefixes as defined when adding the paths:

Once the class is loaded, we can now instantiate it.

Note: In some cases, you may use the same prefix for multiple paths. Zend_Loader_PluginLoader actually registers an array of paths for each given prefix; the last one registered will be the first one checked. This is particularly useful if you are utilizing incubator components.

Note: Paths may be defined at instantiation
You may optionally provide an array of prefix / path pairs (or prefix / paths -- plural paths are allowed) as a parameter to the constructor:

1. $loader = new Zend_Loader_PluginLoader(array(
2.     'Zend_View_Helper' => 'Zend/View/Helper/',
3.     'Foo_View_Helper'  => 'application/modules/foo/views/helpers',
4.     'Bar_View_Helper'  => 'application/modules/bar/views/helpers'
5. ));

Zend_Loader_PluginLoader also optionally allows you to share plugins across plugin-aware objects, without needing to utilize a singleton instance. It does so via a static registry. Indicate the registry name at instantiation as the second parameter to the constructor:

Other components that instantiate the PluginLoader using the same registry name will then have access to already loaded paths and plugins.

The example in the previous section shows how to add paths to a plugin loader. What if you want to determine the paths already loaded, or remove one or more?

• getPaths($prefix = null) returns all paths as prefix / path pairs if no $prefix is provided, or just the paths registered for a given prefix if a $prefix is present.

• clearPaths($prefix = null) will clear all registered paths by default, or only those associated with a given prefix, if the $prefix is provided and present in the stack.

• removePrefixPath($prefix, $path = null) allows you to selectively remove a specific path associated with a given prefix. If no $path is provided, all paths for that prefix are removed. If a $path is provided and exists for that prefix, only that path will be removed.

Sometimes you simply want to determine if a plugin class has been loaded before you perform an action. isLoaded() takes a plugin name, and returns the status.

Another common use case for the PluginLoader is to determine fully qualified plugin class names of loaded classes; getClassName() provides this functionality. Typically, this would be used in conjunction with isLoaded():

Plugin loading can be an expensive operation. At its heart, it needs to loop through each prefix, then each path on the prefix, until it finds a file that matches -- and which defines the class expected. In cases where the file exists but does not define the class, an error will be added to the PHP error stack, which is also an expensive operation. The question then turns to: how can you keep the flexibility of plugins and also address performance?

Zend_Loader_PluginLoader offers an opt-in feature for just this situation, a class file include cache. When enabled, it will create a file that contains all successful includes which you can then call from your bootstrap. Using this strategy, you can greatly improve the performance of your production servers.