Question

im looking at the MVC pattern, and I can see in one example on phppatterns they’re passing the model by reference – any benefit of doing this over a global var? Am I missing something obvious?

class MyView extends View {
  var $model; 

  function __construct(&$model){
    $this->model =& $model;
  }

  function productTable($rownum=1) {
    $rowsperpage='20';
    $this->model->listProducts($rownum,$rowsperpage);
    while ( $product=$this->model->getProduct() ) {
         // Bind data to HTML
    }
  }
}

Any reason why you would do this as apposed to using a global variable? i.e.

class MyView extends View {
  global $model;

  function __construct(){ }

  function productTable($rownum=1) {
    $rowsperpage='20';
    $model->listProducts($rownum,$rowsperpage);
    while ( $product=$this->model->getProduct() ) {
         // Bind data to HTML
   }
}

Answer 1

The problem with global variables is that:

1. They assume that there is only one implementation of model and view.
2. They assume that there is only one instance of the model and view (you could have several of each in your application).
3. They hide the interdependency between components; your view is very strongly affected by the model, but not having to explicitly pass a model into your view makes this implicit.

For other reasons why globals and singletons are “evil” (i.e. just really a poor design decision that you should never make), read avoid global variables, environment variables, and singletons.