Multiple Check box in edit mode: Undefined index

I have multiple checkbox in a form, such as:

1. Car


2. Bike


3. Plane


4. Boat


5. Walk

User can choose one, two, three, or all of it. At the first time, I tried to choosed just one or two option, when I processed the script there's error statement -> "Undefined index for car", "Undefined index for boat" for option I didn't checked. So I was googled and found the answer with this script: (For the insert script)

$car   =  (isset($_POST['car']))?$_POST['car']:NULL;
$bike  =  (isset($_POST['bike']))?$_POST['bike']:NULL;
$plane =  (isset($_POST['plane']))?$_POST['plane']:NULL;
$boat  =  (isset($_POST['boat']))?$_POST['boat']:NULL;
$walk  =  (isset($_POST['walk']))?$_POST['walk']:NULL;

and it was worked perfectly, but then when I want to update my form with this script: (Update/Edit Form)

    if(isset($_POST['save'])) {

    include('connection.php');

    $car   =  (isset($_POST['car']))?$_POST['car']:NULL;
    $bike  =  (isset($_POST['bike']))?$_POST['bike']:NULL;
    $plane =  (isset($_POST['plane']))?$_POST['plane']:NULL;
    $boat  =  (isset($_POST['boat']))?$_POST['boat']:NULL;
    $walk  =  (isset($_POST['walk']))?$_POST['walk']:NULL;

$update = mysql_query("UPDATE information SET(
car   = (isset($_POST['car']))?$_POST['car']:NULL; 
bike  = (isset($_POST['bike']))?$_POST['bike']:NULL;
plane = (isset($_POST['plane']))?$_POST['plane']:NULL;
boat  = (isset($_POST['boat']))?$_POST['boat']:NULL;
walk  = (isset($_POST['walk']))?$_POST['walk']:NULL;
)")

and there's an error, with the quotation marked, so I've changed it into this script: (Fixing the quotation marked)

    if(isset('$_POST[save]')) {

    include('connection.php');

    $car   =  (isset('$_POST[car]'))?'$_POST[car]':NULL;
    $bike  =  (isset('$_POST[bike]'))?'$_POST[bike]':NULL;
    $plane =  (isset('$_POST[plane]'))?'$_POST[plane]':NULL;
    $boat  =  (isset('$_POST[boat]'))?'$_POST[boat]':NULL;
    $walk  =  (isset('$_POST[walk]'))?'$_POST[walk]':NULL;

    $update = mysql_query("UPDATE information SET(
    car   = (isset('$_POST[car]'))?'$_POST[car]':NULL; 
    bike  = (isset('$_POST[bike]'))?'$_POST[bike]':NULL;
    plane = (isset('$_POST[plane]'))?'$_POST[plane]':NULL;
    boat  = (isset('$_POST[boat]'))?'$_POST[boat]':NULL;
    walk  = (isset('$_POST[walk]'))?'$_POST[walk]':NULL;
    )")

if($update){

echo 'Update Success! ';        
echo '<a href="index.php">Results</a>'; 
    }else{

echo 'Failed To Update! ';      
echo '<a href="index.php">Home</a>';    

    }

}else{  

echo '<script>window.history.back()</script>';

Still got error's "Undefined index" but then I was found a solution from stackoverflow also, like this:

function checkbox_value($name) {
    return (isset($_POST[$name]) ? 1 : 0);
}

$sql =  'UPDATE table SET '.
        'checkbox1 = '. checkbox_value('checkbox1') .','.
        'checkbox2 = '. checkbox_value('checkbox2') .','.
        'checkbox3 = '. checkbox_value('checkbox3') .','.
        'checkbox4 = '. checkbox_value('checkbox4') .','.
        'checkbox5 = '. checkbox_value('checkbox5') .','. "LIMIT 1";

I've made changes into this/like this:

function checkbox_value($update) {
    return (isset($_POST[$update]) ? 1 : 0);
}

$update =  'UPDATE information SET '.
        'car = '. checkbox_value('car') .','.
        'bike = '. checkbox_value('bike') .','.
        'plane = '. checkbox_value('plane') .','.
        'boat = '. checkbox_value('boat') .','.
        'walk = '. checkbox_value('walk') .','. "LIMIT 1";

The "Undefined index" was gone, but Failed to update the data. Little bit confusing here...anyone can give me a suggestion please. Thank you very much for the trouble.