Select Point in Polyline Postgis

[anhtin]

hi all
I am developing a web application. i am using Mapserver and Postgis 
I have a question below
if i want to click one point on map to select the nearest point on the
nearest polyline from the point i clicked. How can i do?
Notes: The point I clicked is converted to X,Y coordinate (295149 2315499)

could you show me the script or function on Mapserver.
1. How can i select the nearest  polyline on the click point.
2. How can  i select the the nearest point on the selected nearest polyline.



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