Route tracking on google map php

i have to make a software which is track a vehicle on Google map using GPS. I have almost done this project but i am stuck at a point, the client require a system that track the bus in a route. I am done route but how can i track vehicle on the route or out of route, i look few example like post GIS or spatial Index on MYSQL but i don't know where to start

In MYSQL i am create a linestring my database structure is

Field, Type, Null, Key, Default, Extra id,int(11),NO,PRI,NULL,auto_increment route_name,varchar(45),YES,,NULL, from_id,varchar(45),YES,,NULL, to_id,varchar(45),YES,,NULL, line_string,geometry,NO,MUL,NULL,

where a line_string column is geometric column where i am store my all route coordinates now i have a lat long which is locate on this line or not have to calculate this