//Indexes >>        012345 
String toReverse;
String reversed;
int i;
int j;
int charCount;

void reverse_function() {
  j = charCount -1;
  for (i = 0; i <charCount; i++) {
    reversed = reversed + toReverse[j];
    j--;
  }
}

void setup() {

Serial.begin(9600);
Serial.println("BEGIN");
Serial.println();

}

void loop() {

toReverse = "xy1234";
charCount = toReverse.length();

reverse_function();

Serial.println("To reverse");
Serial.println(toReverse);
Serial.println("Reversed");
Serial.println(reversed);

delay(100000);
}