invalid types of char int for array subscript

[cplusbeginner]

I'm supposed to write a program that if you type a phrase, it outputs the letters used in alphabetical order.
Example: I say hi
it would output
1 a
1 h
2 i
1 s
1 y

When I run the code, it gives me 3 errors of the same message.
invalid types of char int for array subscript.

#include <iostream>
#include <cstring>

using namespace std;

void input(char phrase);
void alphabetical(int letters[], char phrase);
void output(int letters[]);

int main()
{
    char phrase;
    int letters[26];

    input(phrase);
    alphabetical(letters, phrase);
    output(letters);
}

void input(char phrase)
{
    cout << "Enter anything that you want to write." << endl;
    cin.get(phrase);
}

void alphabetical(int letters[], char phrase)
{
    int i = 0, count = 1;
    for(i=0; i <26; i++)
    letters[i] = 0;
    while(phrase[i] != '\0')
    {
        if (phrase[i] == ' ')
            count ++;
        letters[tolower(phrase[i]-97]++;
        i++;
    }
}

void output(int letters[])
{
    int i =0, count = 1;
    cout << count << " words" << endl;
    for(i=0; i<26; i++)
    {
        if(letters[i]>0)
            cout << letters[i] << " " << static_cast<char>(i+97) << endl;
    }
}

[Yanson]

phrase isn't an array so phrase[i] is illegal