ขึ้น Warning: implode() [function.implode]: Invalid arguments passed in C:\AppServ\www\

if(is_array($_POST["txtComputer"]))
     $skillComputer = implode(",", $_POST["txtComputer"]);
else 
     $skillComputer =  $_POST["txtComputer"];

if(is_array($_POST["txtDriver"]))
     $driver = implode(",", $_POST["txtDriver"]);
else 
     $driver = $_POST["txtDriver"];


if(is_array($_POST["txtVehicle"]))
     $vehicle = implode(",", $_POST["txtVehicle"]);
else
     $vehicle = $_POST["txtVehicle"];