Create workout files in .fit format using PHP -- 2

I'm working on an online endurance sports training platform for cycling/ running/ triathlon. I have workouts in an xml format (e.g. warm up at 50% pace for 10mins, then 1min at 100% pace, then 2 mins recovery at 50%, repeat 5 times, then cool down 50% pace). I'd like to take these workouts and create .fit files that users could use on their Garmin (or similar) to do the workout. New workouts are being created all the time, so I need to be able to easily change the xml into fit. It can run locally, doesn't need to be on a web server. I've attached a sample xml file. The intensity/pace is based on Power which is the best Wattage the athlete can hold for an hour, e.g. 300W, as defined by the user. If you think you can do it, try to give me a quote for how long you will need to get it done. Sample XML file: <workout_file> <author>[login to view URL]</author> <name>SE 5X5 bursts and sweet spot</name> <description>SE 5 x (4mins at 60rpm Z3 then 1 minute at 120rpm at Z4), 5 mins rest between each, then 1X10mins sweet spot</description> <sportType>bike</sportType> <tags> <tag name="SE,"/> <tag name="low"/> <tag name="cadence"/> </tags> <workout> <FreeRide Duration="300" FlatRoad="1"/> <SteadyState Duration="120.00001" Power="0.5" pace="1122314923"/> <SteadyState Duration="240.00002" Power="0.78924245" pace="1122314923"/> <SteadyState Duration="60" Power="0.90907824" pace="1122314923" Cadence="120"/> <SteadyState Duration="300" Power="0.5" pace="1122314923"/> <SteadyState Duration="240.00002" Power="0.78924245" pace="1122314923"/> <SteadyState Duration="60.000004" Power="0.91045457" pace="1122314923" Cadence="120"/> <SteadyState Duration="300" Power="0.5" pace="1122314923"/> <SteadyState Duration="240" Power="0.78896654" pace="1122314923"/> <SteadyState Duration="60.000004" Power="0.91045457" pace="1122314923" Cadence="120"/> <SteadyState Duration="300" Power="0.5" pace="1122314923"/> <SteadyState Duration="240.00002" Power="0.78924245" pace="1122314923"/> <SteadyState Duration="60.000004" Power="0.91045457" pace="1122314923" Cadence="120"/> <SteadyState Duration="300" Power="0.5" pace="1122314923"/> <SteadyState Duration="240.00002" Power="0.78924245" pace="1122314923"/> <SteadyState Duration="60" Power="0.91045457" pace="1122314923"/> <SteadyState Duration="300" Power="0.50132352" pace="1122314923"/> <SteadyState Duration="600" Power="0.84984851" pace="1122314923"/> <SteadyState Duration="120" Power="0.50136364" pace="1122314923"/> <FreeRide Duration="180" FlatRoad="1"/> </workout> </workout_file>