THE OBJECTIVE:
for my project, i'd like to install a sonar sensor in front of the car.
once an object (obstacle or wall) detected, then the car stops.
If that is your only objective you can probably do it with much simpler and cheaper solutions. The simplest way, although not as elegant, is to just just a touch switch attached to your front (or rear) bumper. Whenever the car hits an object, the switch will engage and you can use any digital ports to read that information out.
The PING sensor (more generically the ultrasound range sensor) can be used for more sophisticated tasks such as distance measurement, etc...