Find elements in an sorted array $a with a specific distance $k [recursive version]

findElementsByDistance.php

/**
 * Find elements in an sorted array $a with a specific distance $k
 *
 * @param  integer  $k
 * @param  array    $a
 * @param  integer  $i
 * @param  integer  $j
 * @return array
 */
public static function recfindElementsByDistance($k, array $a = [], $i = 0, $j = 1)
{
    $length = count($a);

    if ($i >= $length || $j >= $length)
    {
        return [];
    }
    elseif ($k === $a[$j] - $a[$i])
    {
        return [
            $i => $a[$i],
            $j => $a[$j]
        ];
    }
    else if ($a[$j] - $a[$i] < $k)
    {
        $j ++;
        return self::recfindElementsByDistance($k, $a, $i, $j);
    }
    else
    {
        $i ++;
        return self::recfindElementsByDistance($k, $a, $i, $j);
    }
}