Variable Variables in PHP

The term variable variables comes from the PHP manual (read it.)

When I describe code as dynamic I’m referring to the concept of having code which generates other code during execution.

My reasons for writing about Variable Variables in PHP:

  1. The topic is similar to other obscure PHP features (such as that of calling functions dynamically.)
  2. Implementing variable variables differs syntactically when dealing with OOP — as opposed to in procedural code.
  3. Some PHP design patterns use it.
  4. To understand the CI_Controller class.

Here’s a procedural example:

$a = 'hello';
$$a = 'world';
echo "$a ${$a}"; // output: hello world
echo "$a $hello"; // output: hello world

Here’s an OOP example:

class CI_Controller {

	private static $instance;

	 * Constructor
	public function __construct()
		self::$instance =& $this;
		// Assign all the class objects that were instantiated by the
		// bootstrap file (CodeIgniter.php) to local class variables
		// so that CI can run as one big super object.
		foreach (is_loaded() as $var => $class)
			$this->$var =& load_class($class);

		$this->load =& load_class('Loader', 'core');

		log_message('debug', "Controller Class Initialized");

	public static function &get_instance()
		return self::$instance;

The line which uses variable variables is:

$this->$var =& load_class($class);

Notice it’s creating a new property for the object. The class to which this object belongs does not have that property.


About samehramzylabib

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This entry was posted in PHP Data Value Types, PHP Language Constructs. Bookmark the permalink.


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