Following are Golden rules for successful Projects.
1. Create New repository for new project.
2. Follow SDLC and STLC life cycle for your new project.
3. Always escape your output, never full trust on data in database, Its very good habit to escape your data before displaying in web page.
4. Always filter your input, when user fill data in form do sanitize your data before feeding in database.
5. Use unicode, So that you can display data on web page, specially when using multi language.
6. Use cURL in PHP, Don't use file_get_contents use CURL(Client URL).
7. Coding Convention, Its good to follow coding convention because it will increase readability, understanding and reusability.
8. Don't code when you don't need, today lot of good code snippets available on internet,just need few searches. So Its good to do little R&D before starting work. It will definitely save your precious time.
10. Use JSON in-stead of XML, Today, million of APIs are using JSON because of its quality like fast, human readable etc.
11. Add Security check(Auth/ACL) on every page of website and add Security check from server end also.
12. Developer should work under experienced project manager/Team Lead and He/She should do the Reviews of project weekly.