int myIntVariable = 12345678;
String myStringVariable;

void setup() {

  Serial.begin(115200);

}

void loop() {

//we just need to convert int variable to String and then process that as usual
myStringVariable = myIntVariable;

//indexOf("") locates a character or String within String variable.
//If it does not fing anything, it returns -1
if (myStringVariable.indexOf("78") != -1){
  Serial.println("Contains 78");
}

delay(1000);

}