The function replaces each numeric digit in the input string with its English word equivalent and ignores any letters and special characters.

[CODE]: 
```python
def approx_function(input_string):
    number_to_word = {
        "0": "zero", "1": "one", "2": "two", "3": "three", "4": "four",
        "5": "five", "6": "six", "7": "seven", "8": "eight", "9": "nine"
    }
    output_string = ""
    for char in input_string:
        if char in number_to_word:
            output_string += number_to_word[char]
        else:
            continue
    return output_string
```