$a = array('w','x','y','z');

function iter($num,$arr,$max = 3){
        if( $num == 0 )return;
        $count = count($arr);
        $carr = $arr;
        array_splice($carr,$max,1);
        echo implode(",",$carr)."<br />";
        $max == $count - 1 ? $max = 0 : $max++;
        iter(--$num,$arr,$max);
}

iter(10,$a);