[RajatPawar]

pawn Код:

public OnPlayerStateChange(playerid, newstate, oldstate)
{
    if(newstate == PLAYER_ONFOOT && oldstate == PLAYER_DRIVER)
            // Stop the radio
   else if(newstate == PLAYER_DRIVER && oldstate == PLAYER_ONFOOT)
          // Start it
   return 1;
}

I didn't remember the states, so check them on the wiki again.