How many variable checks should you do? [closed]

Question

Something I've never been sure about is how many variable checks to do in PHP. For example take the following piece of code. I am not checking any of the variables before I assign them or pass them to a function to see if they contain what I expect

$carId = '12';
$aCar = fetchCar($carId);

$make = $aCar['make'];
$model = $aCar['model'];
$yearMade = $aCar['year'];
$age = calcAge($yearMade);

Now if I add some checks

$carId = '12';

if(is_numeric($carId))
{
    $aCar = fetchCar($carId);

    if(isset($aCar['make']) && is_string($aCar['make']))
    {
        $make = $aCar['make'];
    }
    else
    {
        //Report error
    }

    if(isset($aCar['model']) && is_string($aCar['model']))
    {
       $model = $aCar['model'];
    }
    else
    {
        //Report error
    }

    if(isset($aCar['year']) && is_numeric($aCar['year']))
    {
        $yearMade = $aCar['year'];
        $age = calcAge($yearMade);
    }
    else
    {
        //Report error
    }
}
else
{
    //Report errors
}

The code is now better but is it a bit too excessive and bloated? Should I be doing this many checks?

If I shouldn't be doing this many checks where do you draw the line between what you should and shouldn't check?

Answer 1

This is the dilemma of a dynamic type language. It depends heavily on what fetchCar() function is doing.

The approach i would take is assume fetchCar is returning a car array or throwing exception. If you combine this with good exception handling logic you can end up with clean and stable code.

For example:

function fetchCar($id) {

    $car = queryDatabaseSomehow();
    if (empty($car)) {
        throw new ExceptionNotFound();
    }
    //eventually you can put your type checking here?
    if (!isset($car['x']) || !is_string($car['x'])) {
        throw new ExceptionDb();
    }
}

echo fetchCar(3)['make'];

Also if you would like to do this super-proper and go fully OOP, Car should become a class with make,model and year as its members. fetchCar() would return Car or throw Exception. But this is not always desirable of course.