Phly, boy, phly
the weblog and site of Matthew Weier O'Phinney

Closures, Anonymous Functions, and Lambdas, oh my!

Briefly, these are all synonyms (with slight contextual differences) for a single PHP construct, the anonymous function:

You can assign an anonymous function to a variable, or pass it in-line as a callback argument to a function or method call. The construct makes for some really flexible designs, and is particularly useful with the various array functions and with preg_replace_callback(). If you see any "create_function" constructs in your codebase, go and replace them immediately with anonymous functions; not only will they be easier to read (escaping code content in create_function() was always a pain), but they'll be much faster, and also benefit from opcode caching if available.

We discovered one interesting issue, however. PHP does not like serializing closures; doing so raises an exception ("Serialization of 'Closure' is not allowed"). This has a number of implications:

• If you need to alter the SPL autoloader stack, be careful about using closures with it. As an example, our testbed was caching the autoloaders by storing the return value of spl_autoload_functions(), and then resetting it during testing. Unfortunately, if you register a closure with spl_autoload_register, you may get an error when you do this. (Note: this appears to be fixed with 5.3.2 and up.)
• If you are serializing classes that have properties that reference closures, you will need to add some logic to __sleep() and __wakeup() to ensure those properties are not serialized, and to recreate them on deserialization.

Additionally, even though internally anonymous functions are represented via the class Closure, you cannot typehint on that class; the only way to test if a variable is a closure is to use is_callable().

Invokables

One fun new feature of PHP 5 is the magic method __invoke(), which allows you to call an object as if it were a function:

Unlike other magic methods, it actually is faster than the alternatives. When simply returning a value, it's 25% faster than calling a method on the same object; when used with call_user_func_array(), it's 30% faster than using a normal, array-style callback (e.g., array($o, 'greet') -- even when it's proxying to another method!

So, sounds like a great new feature, right? Yes... but there are some things you should know.

• Like closures, you cannot typehint explicitly for __invoke(); you have to either use is_callable() or create an interface defining it:
  
  interface Filter
  {
      public function filter($value);
  }
  
  interface CallableFilter
  {
      public function __invoke($value);
  }
  
  class IntFilter implements Filter, CallableFilter
  {
      public function filter($value)
      {
          return (int) $value;
      }
  
      public function __invoke($value)
      {
          return $this->filter($value);
      }
  }
  
  $filter = new IntFilter;
  if ($filter instanceof CallableFilter) {
      // matches
  }
   
• Be careful about using objects implementing __invoke() as object properties; they don't do what you expect. For instance, consider the following:
  
  class Foo
  {
      public function __invoke()
      {
          return 'foo';
      }
  }
  
  class Bar
  {
      public $foo;
  
      public function __construct()
      {
          $this->foo = new Foo;
      }
  }
  
  $bar = new Bar;
  echo $bar->foo();
   
  You might expect this to echo "foo" -- but it won't. Instead, it'll raise an E_FATAL, claiming "Call to undefined method Bar::foo()". If you want to execute the property, you have to assign it to a temporary variable first, or explicitly call __invoke():
  
  $foo = $bar->foo;
  echo $foo();
  
  // or:
  
  $bar->foo->__invoke();
   

Namespacing for fun and profit

Please put aside your opinions on the choice of namespace separator in PHP; it's water under the bridge at this point, and there were good technical reasons for the choice. We have an implementation, so let's use it.

First off, you declare your namespace at the top of your file:

Or you can have several namespaces in the same file, as long as you have no loose code:

While the above is valid, the PHP manual recommends using braces if you're using multiple namespaces in a single file:

You can import code from other namespaces using the use construct. This construct also allows you to alias the namespace (or class, constant, or function within the namespace) using the as modifier:

Some quick rules about namespaces:

• Fully qualified namespaces (FQN) begin with a namespace separator ("\"). Classes, functions, constants, and static members referenced using a FQN will always resolve.
• The namespace declaration is always considered fully qualified, and should not be prefixed with a namespace separator.
• Namespaces referenced in a use statement are always considered fully qualfied; you can prefix with a namespace separator, but it's not necessary.
• When referring to namespaced classes within a namespace, be aware of the origin: if you don't fully qualify the namespace, the assumptions will be:
  • A sub-namespace of the current namespace
  • A reference to one of the aliases defined when importing
  For example, consider the following code:
  
  namespace Foo;
  use Zend\Filter; // imports are always considered FQN
  
  $foo       = new Bar\Baz;             // actual; Foo\Bar\Baz
  $filter    = new Filter\Int;          // actual; Zend\Filter\Int
  $validator = new Zend\Validator\Int;  // actual: Foo\Zend\Validator\Int
  $validator = new \Zend\Validator\Int; // actual: Zend\Validator\Int
   

One discovery we made was that you can have a namespace that shares the same name as an interface of class. As an example:

This discovery has allowed us to define more "top-level" interfaces within components, with concrete implementations in a namespace matching the interface. This reduces some verbiage, defines a better class hierarchy, and makes the code relations more semantic.

Finally, we've found that one huge benefit to namespaces is when unit testing: we can define a separate namespace for unit tests, as well as separate namespaces for each component. If we use these namespaces for test artifacts -- classes and mock adapters consumed by the unit tests -- we ensure that each test suite is fully encapsulated. This has led to fewer issues with naming collisions.

In closing...

PHP 5.3 offers a ton of new features -- those I go through here are but some of the more prominent ones. If you haven't started hacking with 5.3, you should -- it's definitely the future of PHP, and you'll be seeing an increasing number of libraries and frameworks using it.