PHP Developer for Minor Code Modifications -- 2

I'm in need of a skilled Core PHP developer to assist with minor code changes in my project. The tasks specifically include: - Modifying the existing code for file handling, including enabling file uploads and downloads. - Integrating user authentication feature into the website - Implementing payment gateway and database integration functionalities Ideal candidates should have stellar proficiency in PHP development and be comfortable working with back-end logic. Good command over database related operations is a plus. Serious candidates only, please. Code changes have to be made on code which is on PHP 5.3