Smallest encryption program in C

<?php
$example = range(0, 9);
foreach ($example as $value)
{
 echo $value;
}

int main(int argc, char** argv)
{   
    // retrieve the first argument passed to the program (action)
    char action = argv[1][0];

    // retrieve the second argument passed to the program (key)
    char* key = argv[2];

    // initialize character position in the key
    int i = 0;

    // initialize the current input character
    int c = 0;

    // loop until we reach the end of input
    while (c != -1){
        // get a character from stdin
        c = getchar();
        if (action == 'E'){
            // encode the current character
            putchar(c + key[i]);
        } else{
            // decode the current character
            putchar(c - key[i]);
        }
        // increment the position in the key string, without overflow
        i = (i + 1) % strlen(key);
    }
}