MonoBehaviour.OnApplicationQuit()
Description

Sent to all game objects before the application is quit.

In the editor this is called when the user stops playmode. In the web player it is called when the web view is closed.
	function OnApplicationQuit() {
		// Make sure prefs are saved before quitting.
		PlayerPrefs.Save();
	}
using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void OnApplicationQuit() {
        PlayerPrefs.Save();
    }
}
import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def OnApplicationQuit() as void:
		PlayerPrefs.Save()

Note that iOS applications are usually suspended and do not quit. You should tick "Exit on Suspend" in Player settings for iOS builds to cause the game to quit and not suspend, otherwise you may not see this call. If "Exit on Suspend" is not ticked then you will see calls to OnApplicationPause instead.