The function takes a string input, replaces the second character with a capital "P", and reverses the substring between the second character and the second-to-last character. The first and last characters remain unchanged.

[CODE]:
```python
def approx_function(s):
    if len(s) < 2:
        return s

    middle_substring = s[2:-1]
    middle_substring_reversed = middle_substring[::-1]

    return s[0] + "P" + middle_substring_reversed + s[-1]
```

Please note that this approximation might not cover more specific edge-cases. To get a perfect replica, we would need to analyze the original function source code.