Senior Full-Stack PHP Developer — Legacy E-Commerce Platform

About the Company We are an established Japan-based international e-commerce company serving customers across multiple countries. Our business operates through a large custom-built platform containing a substantial product catalogue, customer accounts, inventory feeds, inquiries, transactions and internal administrative functions. The current application is a large custom PHP system developed over many years. It includes: Procedural PHP without a framework MySQL/MariaDB A custom administration panel Large-scale inventory and product-data feeds Customer registration and inquiry functions Transaction and operational management tools This is not a greenfield project. You must be comfortable understanding and improving a large legacy application while keeping the live business stable. We are looking for one dependable developer who can learn the system deeply, take long-term ownership of development work and gradually help us modernize the platform. Responsibilities Develop and maintain functionality across PHP, MySQL, HTML, CSS and JavaScript. Diagnose and fix issues within the existing legacy PHP application. Improve automated product and inventory imports using SFTP/FTP, CSV, XML and third-party feeds. Monitor and improve data accuracy and import reliability. Extend the custom administration panel and internal operational tools. Maintain customer registration, contact-form and inquiry workflows. Integrate website registrations and inquiries with our CRM. Implement technical SEO improvements at scale, including canonical URLs, hreflang, XML sitemaps and programmatic pages. Improve website speed, caching and Core Web Vitals. Identify and resolve security risks, including hardcoded credentials, SQL-injection risks, exposed endpoints and access-control weaknesses. Move application configuration and credentials into secure environment-based configuration. Investigate production issues and deliver fixes without disrupting the live business. Participate in code reviews and follow a pull-request-based deployment process. Document important systems, integrations and technical decisions. Establish better development patterns for new code. Support a gradual migration toward a modern PHP framework and architecture. Required Experience At least 5 years of professional PHP development experience. Significant experience working with large legacy PHP applications. Comfortable working with procedural PHP where business logic and HTML may be mixed. Strong MySQL/MariaDB knowledge, including writing and optimizing raw SQL. Experience working without an ORM. Strong understanding of prepared statements, authentication, authorization, secrets management and secure coding. Good working knowledge of HTML, CSS, vanilla JavaScript and jQuery. Experience using Git and pull-request-based workflows. Ability to deploy changes safely where automated test coverage is limited. Strong troubleshooting and debugging skills. Clear written English and reliable asynchronous communication. Ability to work independently and provide transparent progress updates. Preferred Experience Modern PHP practices, including Composer, PSR standards and PSR-4 autoloading. Laravel, Symfony or another modern PHP framework. Gradual modernization or migration of legacy applications. Cron-based batch processing and third-party data feeds. Technical SEO for large e-commerce, catalogue or marketplace websites. AWS S3, CloudFront or another CDN. Image processing using GD or ImageMagick. PDF and Excel generation. Linux, cPanel and VPS administration. CI/CD using Bitbucket Pipelines. CRM integrations, especially Kommo CRM. International e-commerce or marketplace experience. Technology Stack PHP 8.2 without a framework MySQL/MariaDB mysqli and raw SQL HTML, CSS, JavaScript and jQuery Bootstrap-era CSS Include-based PHP templates and partials Composer AWS S3 TCPDF PHPMailer Bitbucket and Bitbucket Pipelines Linux/cPanel VPS Cloudflare Working Arrangement You will work directly with the Company Director and our Fractional CTO. The Fractional CTO will participate in applicant screening, conduct the technical evaluation, prioritize development work, review code and guide the platform’s modernization. We are seeking a stable, long-term working relationship with someone who will understand the platform and take ownership of it. Confidential information about the company, platform and codebase will be shared only with shortlisted applicants when appropriate. Hiring Process Shortlisted applicants will complete: An initial profile and experience review. A technical interview with our Fractional CTO. A short paid trial based on a real, carefully scoped development ticket. The paid trial will evaluate how you: Understand an unfamiliar legacy system Investigate existing code Communicate risks and assumptions Make safe and focused changes Test and document your work Avoid unnecessary rewrites How to Apply Please apply only through Upwork and include: Your CV or a summary of your relevant experience. Two examples of legacy PHP systems you personally improved. For each example, explain: What you inherited What problems you identified What you changed How you tested your changes How you avoided disrupting the live system Your experience working with raw PHP and MySQL without a framework. Your experience modernizing legacy applications. Your weekly availability and normal working hours. How many hours you can overlap with 13:00–18:00 JST. Your expected hourly rate. Your earliest available start date. Please do not attempt to contact the company outside Upwork. All applications and initial communication must remain on Upwork. Generic applications that do not address these questions may not be considered.