Sorting multidim array: prioritize if column contains substring, then order by a second column

I am currently creating a sorting method that consists of values from an mysql query.

Here's a brief view of the array:

    Array
    (
        [0] => Array
            (
                ['id'] = 1;
                ['countries'] = 'EN,CH,SP';
            )
        [1] => Array
            (
                ['id'] = 2;
                ['countries'] = 'GE,SP,SV';
            )
    )

I have succeeded in making a normal usort based on the numeric id values, but I rather want to sort the array by the content of the "countries" field (if it contains a set string, a country code in this case), and then by the id field.

The following snippet was my first idea of how to do it, but I have no idea of how to incorporate it into an working function:

in_array('EN', explode(",",$a['countries']) );

How would you do it?

Thanks!

I am really getting nowhere with this unfortunately.

Here is what I have for the moment, and its giving me nothing but errors: uasort() [function.uasort]: Invalid comparison function

function compare($a, $b) {
    global $usercountry;

        if ( in_array($usercountry, $a['countries']) && in_array($usercountry, $a['countries']) ) {
            $return = 0;
        }

        else if (in_array($usercountry, $a['countries'])) {
            $return = 1;
        }

        else {
            $return = -1;
        }

        return $return;


        }

        $array= usort($array, "compare");

Is there anyone who might give me a hint of how to go on with it?