In many cases you may need to take in extra information from user and display some type of graph after registration which cannot be fulfilled by adding extra fields within the form. In other way we can redirect to a second page containing your required form.
In many cases of theming in Drupal we may may need a different set of header and footer. We can achieve this by writing a couple of lines in template.php for your current Drupal theme. Code snippet for Drupal 7 node type based custom template.
Many people have heard about PHP 5, but how much they have applied in their real life is my question.