The first step in developing every program’s business, right after designing and defining the required classes, is class instantiation. And it is a very straightforward process. You call the class constructor which is exactly the same as the class name, followed by the respective namespace, pass the required arguments (if any), and that’s it.

The scenario described above is exactly what happens in nearly all situations. But there is a condition where exceptions may arise: whenever you try to instantiate a class dynamically.

So, what is dynamic class instantiation/loading?

Dynamic Class Instantiation allows the loading of code that is not known about before a program starts. This way, the program doesn’t need to be aware of the class name before it's loaded and run.

class BicycleModel{
  //@ToDo: implement BicycleModel
}

class CarModel{
  //@ToDo: implement CarModel
}

// Normal Class Loading
$myTodayChoice = new BicycleModel();

// Dynamic Class Loading
$vehicleName = 'Bicycle';
// You should first concatenate $vehicleName & Model, then use resulting variable for creating class instance.
// Keep in mind that you're not allowed to merge class instantiation & concatenation.
// $myTodayChoice - new $vehicleName.'Model'(); IS NOT VALID
$className = $vehicleName . 'Model';
$myTodayChoice = new $className();

As you see, we first specify the desired class name in the flow of the program and then use it to declare the class. 

So far, what we’ve discussed is occurring in a global scope. Now its time to dig into Dynamic Class Loading in Namespaced PHP.

It seems there is nothing new to worry about: we’ll just declare some namespaces, then every package and class has to be defined in its proper namespace. So we just have to follow the rules about normal class loading. Let's do it and see what happens:

/* File Component/Package/Vehicle/BicycleModel.php */

namespace Component\Package\Vehicle;

class BicycleModel{
  //@ToDo: implement BicycleModel
}
------------------------------------------------------

/* File Component/Package/Vehicle/CarModel.php */

namespace Component\Package\Vehicle;

class CarModel{
  //@ToDo: implement CarModel
}
------------------------------------------------------

/* File Component/Package/Vehicle/VehicleFactory.php */

namespace Component\Package\Vehicle;

class VehicleFactory
{
  private $vehicleName;

  public function __construct($vehicle = 'Bicycle'){
    $this->$vehicleName = $vehicle;
  }

  public function getInstance(){
    $className = $this->$vehicleName . 'Model';
    return new $className(); //This will return \BicycleModel & NOT \Component\Package\Vehicle\BicycleModel
    /* This means it won't work as expected. We thought it will return a BicycleModel in the same namespace
    *  that is defined, But it didn't happen. Continue reading to see why */
  }
}

Defining namespaces and importing packages from other namespaces is performed at compile-time and so does not affect dynamic class names (which are determined at run-time).

So, in order to fix bug, we have to rewrite the  getInstance() method, prefixing te $className  with the proper namespace:

public function getInstance(){
  $className = 'Component\\Package\\Vehicle\\' . $this->$vehicleName . 'Model';
  return new $className(); //This will return \BicycleModel & NOT \Component\Package\Vehicle\BicycleModel
  /* This means it won't work as expected. We thought it will return a BicycleModel in the same namespace
  *  that is defined, But it didn't happen. Continue reading to see why */
}