Is it a good idea to create dynamic class variables in PHP?

When creating classes, I followed OO conventions and declared all class variables before using them:

    class myClass {
      private $property1, $property2, ...;

      public __constructor() {
         $this->property1 = $this->property2 = NULL;
      }

    }

But I realized that PHP is a scripting language, and not strictly adhere to the concepts of OO, so we can dynamically generate a class property:

    class myClass {
      public __constructor() {
         $this->fields = $this->db->getFields(TABLE_NAME);

         foreach($this->fields as $fld) {
           $this->{$fld} = NULL;
         }
      }

    }

Is this a good approach? I think that dynamically generated properties will have public access by default, so this may be one of the drawbacks, and such automation may be one of the advantages. Is there a difference in performance?