Creating the Singleton design pattern in PHP5

Question

How would one create a Singleton class using PHP5 classes?

Answer 1

to use:

/**
 *  example of class definitions using SingletonTrait
 */
class DBFactory {
  /**
   * we are adding the trait here 
   **/
   use SingletonTrait;

  /**
   * This class will have a single db connection as an example
   **/
  protected $db;


 /**
  * as an example we will create a PDO connection
  **/
  protected function __construct(){
    $this->db = 
        new PDO('mysql:dbname=foodb;port=3305;host=127.0.0.1','foouser','foopass');
  }
}
class DBFactoryChild extends DBFactory {
  /**
   * we repeating the inst so that it will differentiate it
   * from UserFactory singleton
   **/
   protected static $inst = null;
}


/**
 * example of instanciating the classes
 */
$uf0 = DBFactoryChild::getInstance();
var_dump($uf0);
$uf1 = DBFactory::getInstance();
var_dump($uf1);
echo $uf0 === $uf1;

respose:

object(DBFactoryChild)#1 (0) {
}
object(DBFactory)#2 (0) {
}

If you are using PHP 5.4: trait its an option, so you don't have to waste the inheritance hierarchy in order to have the Singleton pattern

and also notice that whether you use traits or extends Singleton class one loose end was to create singleton of child classes if you dont add the following line of code:

   protected static $inst = null;

in the child class

the unexpected result will be:

object(DBFactoryChild)#1 (0) {
}
object(DBFactoryChild)#1 (0) {
}