How to pause an functin with Setinterval and play again?

Little bit stuck here, I call a function changePhoto with setInterval.

Furthermore I got two buttons (pause and play)

Now with pause I just clearInterval and with play I want to start where
changePhoto has been stopped???

nav_mc.pause_mc.onRelease = function()
	trace("Pause this "+pIndex);	
	nav_mc.play_mc._visible =true;
	this._visible = false;
nav_mc.play_mc.onRelease = function()
	trace("Play this");
	nav_mc.pause_mc._visible =true;
	this._visible = false;
playID = setInterval(this, "changePhoto", 4000, 1);