printStr(str_split('012'));
function printStr($arr, $length = 4, $str = '')
{
    if(empty($arr) || !is_array($arr))
    {
        return;
    }
    foreach($arr AS $k=> $v)
    {
        $str    .= $v;
        if(strlen($str)<$length)
        {
            $temp    = $arr;
            unset($temp[$k]);
            printStr($temp, $length, $str);
        }
        else
        {
            echo    $str . '<br />';
        }
        $str    = substr($str, 0, -1);
    }
}