How to get instance of a specific class in PHP?

Question

I need to check if there exists an instance of class_A ,and if there does exist, get that instance.

How to do it in PHP?

As always,I think an simple example is best.

Now my problem has become:

$ins = new class_A();

How to storing the instance in a static member variable of class_A when instantiating?

It'll be better if the instance can be stored when calling __construct().Say,it should work without limitation on how it's instantiated.

Answer 1

if ($object instanceof class_A)

See PHP manual: Classes and objects

Answer 2

You should implement the Singleton pattern. http://www.developertutorials.com/tutorials/php/php-singleton-design-pattern-050729/page1.html

Answer 3

I think what you want is the Registry Pattern

Answer 4

To expand on Pikrass answer, you basically will want to do something like this:

class class_A {
  private static $instance = false;

  public static function getInstance() {
    if (!self::$instance) {
      self::$instance = new class_A();
    }

    return self::$instance;
  }

  // actual class implementation goes here
}


// where you need to use the instance:
$mySingleton = class_A::getInstance();
// do something with $mySingleton

Answer 5

The singleton pattern, with a PHP example from Wikipedia that I've added a "checkExists" method to, as it sounds like you want to check for the existence of the class without necessarily creating it if it doesn't exit:

final class Singleton 
{
    protected static $_instance;

    protected function __construct() # we don't permit an explicit call of the constructor! (like $v = new Singleton())
    { }

    protected function __clone() # we don't permit cloning the singleton (like $x = clone $v)
    { }

    public static function getInstance() 
    {
      if( self::$_instance === NULL ) {
        self::$_instance = new self();
      }
      return self::$_instance;
    }

    public static function checkExists() 
    {
      return self::$_instance;
    }
}

if(Singleton::checkExists())
   $instance = Singleton::getInstance();

Answer 6

Maybe you want something like

for (get_defined_vars() as $key=>$value)
{
  if ($value instanceof class_A)
    return $value;
}

EDIT: Upon further reading, you have to jump through some hoops to get object references. So you might want return $$key; instead of return $value;. Or some other tricks to get a reference to the object.

Answer 7

Remember that by using a singleton, you're basically creating a big global variable. If it's got state that changes, your code can become unpredictable. So use caution.

Answer 8

What you ask for is impossible (Well, perhaps not in a technical sense, but highly impractical). It suggests that you have a deeper misunderstanding about the purpose of objects and classes.

Answer 9

Can you modify the constructor? So you could have:

class a {
    public static $instance;
    public function __construct() {
        self::$instance = $this;
    }
}

if ($a = a::$instance) {
    //do something with existing instance of a
}

Answer 10

If the replacement of the singleton instantiation instrucion in your files is a problem you may turn into a constant-driven behaviour: constants are such for all the duration of the script, so in case of an instance which requirement is to be unique (for all the script duration) the construction method may be properly linked to the existence/value of a constant.

class superObject {
    public function __construct() {
        if (defined('SUPER_OBJECT')) {
            trigger_error('Super object '.__CLASS__.' already instantiated', E_USER_ERROR);
                    // ...or just do whatever you want to do in case of instances overlap
        } else {
            define('SUPER_OBJECT', true);
        }
    // the rest of your construct method
    // ...
    }
}