Merge sorted arrays unique with runtime O(n)

array_merge_unique.php

/**
 * Merge sorted arrays unique
 *
 * Function merges two sorted arrays A & B and produces a new sorted array C
 * with all elements of A and B without repetitions.
 *
 * For instance, if
 *   A = [1, 1, 2, 2, 5, 7, 16] and
 *   B = [2, 5, 10, 10, 11, 12, 14, 16],
 *   then
 *   C = [1, 2, 5, 7, 10, 11, 12, 14, 16].
 *
 * @param  array  $a
 * @param  array  $b
 * @return array
 */
function array_merge_unique(array $a, array $b)
{
    $m   = count($a);
    $n   = count($b);
    $res = array();
    $i   = 0;
    $j   = 0;
    $k   = 0;

    while ($i < $m && $j < $n)
    {
        if ($a[$i] === $b[$j])
        {
            if (0 === $k)
            {
                $res[] = $a[$i];
                $k++;
            }
            elseif ($res[$k - 1] !== $a[$i])
            {
                $res[] = $a[$i];
                $k++;
            }
            $i ++;
            $j ++;
        }
        elseif ($a[$i] < $b[$j])
        {
            if (0 === $k)
            {
                $res[] = $a[$i];
                $k++;
            }
            elseif ($res[$k - 1] !== $a[$i])
            {
                $res[] = $a[$i];
                $k++;
            }
            $i ++;
        }
        else
        {
            if (0 === $k)
            {
                $res[] = $b[$j];
                $k++;
            }
            elseif ($res[$k - 1] !== $b[$j])
            {
                $res[] = $b[$j];
                $k++;
            }
            $j ++;
        }
    }

    for ($i; $i < $m; $i ++)
    {
        if ($res[$k - 1] !== $a[$i])
        {
            $res[] = $a[$i];
            $k++;
        }
        $i++;
    }

    for ($j; $j < $n; $j ++)
    {
        if ($res[$k - 1] !== $b[$j])
        {
            $res[] = $b[$j];
            $k++;
        }
        $j++;
    }

    return $res;
}