I am new to drupal and just finished studying the basic concepts in drupal. We are migrating an existing php site into drupal. The website posts articles every week and the articles are stored in database.
Do I need to create a custom theme if I want to have exactly the same presentation as of existing site?
And just seeking advice that if there is any easier way of migrating an existing dynamic php site into drupal.
You will definitely need to create a custom theme if you want to have it mirror the way your current site looks. I would recommend using a base theme, like Zen, and customize from there.
As far as migration goes, it really depends on how your site is currently setup. I would recommend writing a custom script to migrate your articles into Drupal.