Question

I have 10 sets of 3 values. At each point, I will only know 1 of the values and need to retrieve the other 2 values. For example, I know what id_a2 is, how do I use that to get id_a1 and id_a3?

id_a1, id_a2, id_a3
id_b1, id_b2, id_b3
id_c1, id_c2, id_c3...

Answer 1

Store it in a multi-dimentional array, then loop through it:

$data = array(
    array('id_a1', 'id_a2', 'id_a3'),
    array('id_b1', 'id_b2', 'id_b3'),
    array('id_c1', 'id_c2', 'id_c3')
);

find_siblings('id_b2', $data); // returns: array('id_b1', 'id_b3')

function find_siblings ( $needle, $haystack )
{
    foreach ( $haystack as $array )
    {
        if ( in_array($needle, $array) )
        {
            $out = array();

            foreach ( $array as $value )
            {
                if ( $value != $needle ) $out[] = $value;
            }

            return $out;
        }
    }
}

See it here in action: http://viper-7.com/VPVTnU