Extending plugins in PHP and Symfony

Plugins are great but they are never what you exactly wanted. When they are designed properly, the best way to customize them is to extend them instead of directly editing them.

Now, imagine I have:¹

# Penguin.class.php    
class Penguin          
{                      
  public function __construct()
  { 
    echo "Windows is bad\n";
  }
}

# Herd.class.php
class Herd
{ 
  public function __construct()
  { 
    while ($i++ < 42) new Penguin();
  }
}
 
 
# test
new Herd();

I want to change Penguin‘s behavior to something a bit more positive. However I don’t want to extend Herd especially if it’s tied to Penguin everywhere… I’m stuck with the Penguin class and can’t use another one.

With some languages you can alter classes dynamically, and it can be referred as monkeypatching. While it is actually possible in PHP, it is ugly at best; you end up typing code in character strings, losing proper syntax highlighting and the opcode caching. Unless there is a very good language support for these methods it’s best to avoid them.

A handful of plugins for Symfony, like sfGuardPlugin (one of the most popular plugins) already have some kind of solution. They come with two classes, a dummy one and a real one:

# Penguin.class.php
class Penguin extends pluginPenguin
{
}

# pluginPenguin.class.php
class pluginPenguin
{ 
  public function __construct()
  { 
    echo "Windows is bad\n";
  }
}

Which means you just have to edit the (almost) blank Penguin class.
This is a clean, object-oriented way to solve the problem.

However, there are practicals problems that will arise.

• You still edit an existing file; your modifications would be erased by upgrading to a newer version of the plugin
• It will confuse version control systems if you use one to retrieve the plugin
• Your custom code is in the plugin directory, which is just illogical
• You could chose no to ship any of the blank files, but the user would have to create all of them

There is a very simple solution to overcome all that: the Symfony autoloader will pick classes from local folders (like the lib folder of your project) first.
Which means you can just copy the Penguin.class.php file and customize it:

# Penguin.class.php your project's lib/ folder
class Penguin extends pluginPenguin
{
  public function __construct()
  {
    echo "Linux is good\n";
  }
}

If you don’t use Symfony and/or a similar autoloader, there is another solution:

# Penguin.class.php in the sfHerdPlugin directory
require dirname(__FILE__).'/plugin/plugin'.basename(__FILE__);
 
if (is_readable(dirname(__FILE__).'/../sfHerdPluginCustom/'.basename(__FILE__)):
  require dirname(__FILE__).'/../sfHerdPluginCustom/'.basename(__FILE__);
else:
  class Penguin extends pluginPenguin
  {
  }
endif;

# Penguin.class.php in the sfHerdPluginCustom directory (optional)
class Penguin extends pluginPenguin
{
  public function __construct()
  {
    echo "Linux is good\n";
  }
}

The cool aspect is that if you don’t create any corresponding files in the sfHerdPluginCustom directory, it will still work perfectly.

1. The Herd and Penguin classes are references to GOTO++, a funny programming language. [↩]