How to send array

Projects Networking, Protocols, and Devices
1

char id = 'usa_0001';
char arr[] = {};
....
while (client.connected()) {
char x = digitalRead(LED_BUILTIN);
char arr[] = {id,x};
....
Serial.println(arr);

WHY arr print show "1"?

2

Because.

This is just a snippet of my answer.

3 
char id = 'usa_0001';

Single quotes are for single characters. Post a picture of your keyboard with the usa_0001 key circled, please.

char arr[] = {};

How many elements can this array hold? Any answer other than 0 is wrong.

    char arr[] = {id,x};

Why on earth would you define a new array with the same name as a global array?

Serial.println(arr);

Which array are you printing?