How to show a error page when there is a php mistake?

[eluser]chefnelone[/eluser]
Even though UhOh is very useful and I already integrated to my ci app.
What I was looking for is something to use when the site goes Live so that if I made a php mistake it can show a customized error page, something like "We are doing some maintenance, come back later" which is much better than a blank page.
thanks