Improve User Interface.
MySQL: maintenance, optimization, knowledge of complex queries (JOIN, UNION, subqueries), understanding of InnoDB engine;
PHP: understanding of OOP (classes, objects), being able to work with other developers' code (Wordpress, Drupal are not necessary);
Experience in working in a team using version control systems (GIT preferred);
Linux/Unix: SSH/SFTP, bash commands, Nginx/Apache web server;
English proficiency is not important