PHP action when more than one checkbox was selected

I've got a POST form with its action on the same page. The form has many checkboxes and I'd like run some PHP line when the user has checked more than one checkbox (PHP should run after submitting the form).

So basically I'd need following in PHP:

If checked checkboxes of #FormXY > 1
...do something...
else (which means 0 or 1 checked checbox)
...do something else

Thanks in advance, Tom