Online notes sharing site

I have developed an online notes-sharing platform using PHP and MySQL. I am looking for an experienced PHP developer to help me with:

- Fixing bugs and optimizing database queries: This is the first priority. The immediate concern is ensuring data integrity, particularly with foreign key constraints.

- Improving website performance and security: Once the bugs are fixed and queries optimized, I would like to improve the overall site performance and bolster its security.

- Adding new features: Post bug fixes, I want to implement new features such as user profiles, a search functionality, and the ability for users to upload files.

- Enhancing the UI: Finally, I want to improve the user interface for a better user experience.

Ideal Skills:
- PHP
- MySQL
- HTML, CSS, JavaScript (optional but preferred)
- Experience with website optimization and security

Budget & Timeline: Please provide your estimated budget and time frame for completing the improvements.