In at the deep-end: this Drupal PHP module served small businesses in digital arts and IT services. It allowed them to record their contact details, customer testimonials, categorise their services, and show portfolio descriptions with links and images.
Service providers (members) can update their own details after a regular login. They can choose categories and upload logo image files. Users (who might be businesses with a job to be fulfilled) can search by free-text and category.
A great help in this task was the excellent book "Pro Drupal Development" by John K VanDyk and Matt Westgate. It has real explanations and practical code to show you how to get started. No book tells you everything, but this one is genuinely useful in getting you started.
Naturally this used industrial-strength PHP and MySQL, and we retained an excellent state of HTML validation throughout.