Professional PHP Developer for E-commerce Customization

I'm in need of a skilled PHP developer to modify my E-commerce platform's source code. This project involves adding new features, optimizing the code for performance, and enhancing the security of the application.

Key Responsibilities:
- Implement necessary feature additions to the platform
- Optimize code for peak performance
- Enhance security measures of the application

The ideal candidate should have:
- Deep expertise in PHP, particularly the latest versions
- Experience with MySQL or any compatible database
- Knowledge of JavaScript and jQuery is a plus
- Proven track record in code performance and security enhancements
- Experience with API integration and system connectivity

Most important content management features to implement:
- User roles and permissions
- Media management
- SEO tools

Please provide examples of your previous work and an estimated timeline for the modifications. Thank you.