I want to create a new content type with fields in it using custom PHP module which is present in Drupal 6.
I need to create a new content type using custom PHP, not to add value to a content type.
You can make a module which will create a content type on installation. You can take help of this documentation on drupal.org. A small tutorial is available here too..
Next thing you will want to insert data pro-grammatically & here is the help - Inserting & Updating Node Programmatically
I know it's takes time to start and sometime to make it run but at-least you are willing to start now. So happy coding with Drupal..GOOD LUCK :)
You can use the CCK module to create custom content types in Drupal 6. I'm not sure what you mean by "custom PHP module". CCK should do the trick.