A multi-select should put an array inside the $POST array. I suppose in order to get the format you want, you'd run a loop with a counter variable (1,2,3, etc.), create a new array with the counter as key and the value from the $POST array as the value, and then call [man]implode/man on it using "." as the glue ...
But, curious ... what is the application, and why exactly, does it need it exactly this way? It looks an awful lot like a [man]serialize[/man]d() PHP array, which would be almost "automagic" to create.
HTH,