如何在数组中找到连续的单词

ok, this is driving me insane, the solution must be easy but I've hit the proverbial wall,

here it is:

suppose I have this Array: a, b, c, d

I want to find all the contiguous letters in the array without mixing them, such as:

a b c d
a b c
a b

b c d
b c

c d

I've done many tests, but for some reason I just can't get it right. Any hint would be greatly appreciated.

$arr = Array('a', 'b', 'c', 'd');
for ($i = 0; $i < count($arr); $i++) {
    $s = '';
    for ($j = $i; $j < count($arr); $j++) {
        $s = $s . $arr[$j];
        if (strlen($s) > 1) {
            echo $s.' ';
        }
    }
}

OUTPUT:

ab abc abcd bc bcd cd