[DWIGHTpN]

PHP Code:

public RemoveAllButLetters(buffer[]) {
    new i = 0, j = 0;
    while (buffer[i] != EOS)
        if (isalpha(buffer[i]))
            buffer[j++] = buffer[i++];
        else
            i++;
    buffer[j] = EOS;
    return j;
} 


O(n).