I see that you almost never check the result of mysql_query:
[code=php]mysql_query ("INSERT INTO Employees (name, phone, email, start_working_time,end_working_time,day_off,categories) VALUES ('$name', '$phone', '$email','$start_working_time','$end_working_time','$day_off','$categories')");
// what does this return?
From the documentation:
php.net wrote: For SELECT, SHOW, DESCRIBE, EXPLAIN and other statements returning resultset, mysql_query() returns a resource on success, or FALSE on error.
For other type of SQL statements, INSERT, UPDATE, DELETE, DROP, etc, mysql_query() returns TRUE on success or FALSE on error.