exercism fetch php series


Given a string of digits, output all the contiguous substrings of length n in that string.

For example, the string "49142" has the following 3-digit series:

And the following 4-digit series:

And if you ask for a 6-digit series from a 5-digit string, you deserve whatever you get.

Note that these series are only required to occupy adjacent positions in the input; the digits need not be numerically consecutive.