replacing sub strings in a loop with conditions in mind

Hi! I have a slightly weird code I have to work on and I am hoping someone can give me a hand with this. It is ok if your suggestion is in pseudo code or something other than PHP as long as I get to understand the concept.

The project is complex so I will only expose the part that really troubles me..

I have a huge array or strings. Each string is a series of numbers in a range from 1 to 8,,, and basically the array is comprised of 40320 permutations of the original sequence.

so among many others my strings can look something like this:

$string[0] = "8 6 7 4 2 3 5 1";
$string[1] = "8 6 7 4 2 5 1 3"
$string[2] = "6 5 1 3 2 7 8 4"
$string[3] = "6 5 1 3 2 8 4 7"

what I need to do is iterate through the array and place a DASH between all possible numbers that behave like lower to higher value pairs...

what I am saying is this

IF MY STRING IS

1 2 3 4

then all my possible dashed replacements would look like this

1-2 3-4
1 2-3 4
1 2 3-4

it is important to keep in mind that the same number can not be linked with a dash twice

for example, this is not an option
1-2-3-4

the idea is that if two digits are paired up they can not be paired up with another digit.

also... for this string

6 5 1 3 2 7 8 4

my results would be

6 5 1 3 2 7-8 4

but in a string

1 2 3 4 5 6 7 8

there would be many solutuons...

HELPP!!! (lol,, seriously,, I am completely messed with this)