Restrict function parameter to allow specific values [duplicate]

A number of ways you could do this. The quick and dirty way is to manually check values in the function and process them accordingly:

function myFunction($carType, $isOld, $something)
{
    if (in_array($carType, array('car1', 'car2', 'car3'))) {
        echo 'Car type is valid';
    } 
    if (is_bool($isOld)) {
        echo 'Car age is valid';
    } 
}

The other way is to typehint the function definition with objects that validate the input. PHP doesn't support type hinting for all types so objects may be the way to go but this may be overkill for this situation.

Are you looking for a simple kind of validation for what variable is passed into your function, something like this?

function myFunction ($carType = null, $isOld = null, $something = null)
{
    $carTypeValues = array ("car1", "car2", "car3");
    $isOldValues = array (true,false);

    $strict = true; // strict comparison here? e.g. null == false if $strict is false

    if(
        !$carType || !$isOld                            // variables aren't empty?
        || !in_array($carType, $carTypeValues, $strict) // passed carType is ok?
        || !in_array($isOld, $isOldValues, $strict)     // passed isOld is ok?
    ) {
        // error, incorrect variable passed
        return false;
    }

    //do what is required
    return true;
}

This will allow you to check that A. your variables passed exist, and B. that they exist within an array of acceptable options that you define.

I know I can check the value once I actually execute the function call, however I am looking for something that will allow the user to preferably be able to use selectors

You can't specify selectors like in your example, but you could pass an integer in to represent an array key, thus avoiding the need to duplicate the variable's value:

function myFunction($carType = null, $isOld = null, $something = null) { 
    $carTypeValues = array ("car1", "car2", "car3");
    $isOldValues = array (true,false);  

    if(
        !$carType || $isOld
        || !array_key_exists($carType, $carTypeValues)
        || !array_key_exists($isOld, $isOldValues)
    ) {
        return false;
    }

    // handle OK
    return array(
        'car_type' => $carTypeValues[$carType],
        'is_old'   => $isOldValues[$isOld]
    );
}

myFunction(1, 0, 'foobar'); // array('car_type' => 'car2', 'is_old' => true);

PHP language has no Enum type (proposition only).

But you could obtain splEnum by installing special extension.

PHP and enumerations question