Doofenshmirtz: Love was once a crazy dream,
Now it's my new evil scheme
Doofenshmirtz's Girlfriend: And I'm as happy as can be!
Both: It's the age-old story how an evil boy meets an evil girl,
We got a love strong enough to rule the whole wide world,
We both maniacally laugh at all the same stuff,
I found my other half, yes,
I got an evil love!
Backup Singers: (La-la, la-la-la-la)

Doofenshmirtz: When our robot armies march
Girlfriend: To the beating of our hearts,
I'm as happy as can be!

Both: It's the age-old story how an evil boy meets an evil girl,
We got a love strong enough to rule the whole, wide world,
We both maniacally laugh at all the same stuff,
You can't foil a plan that's built upon evil love!
Backup Singers: (Love-love, love-love-love-love)
Girlfriend: Oohoo
Doofenshmirtz: Evil love...
Girlfriend: Oohoo

http://phineasandferb.wikia.com/wiki/Happy_Evil_Love_Song