PLID isn't a property of CON (the packet), it's a property of CON->A and CON->B (the two carContact structures inside the packet).
If PRISM sets up the object as I would guess it does, you should be able to remove the '$C=...' line, and then replace the above line with the following:
<?php
$playerA = $this->getClientByPLID($CON->A->PLID);
$playerB = $this->getClientByPLID($CON->B->PLID);
$this->SoundMessageToAllPlayers("Car contact: ".$playerA->PName." and ".$playerB->PName);
?>