Can the iPhone 5 charger be used for the iPad mini?
The very short answer is... yes. The chargers that ship with the iPhone 5 and iPad mini 5 watt Lightning port chargers and perfectly interchangeable.
But it's a bit more complicated than that.
The more pertinent question might be, can I use a full size iPad charger with my iPad Mini or iPhone 5? Again, the answer here is yes. Although an iPad 4 charger or standalone charger from the Apple store is 12 watts rather than 5.
The USB to Lightning connector is the important component here, not the charger. The device will only pull as much charge as it is able.
Interestingly, while your iPhone 5 won't draw any more than 5 watts while charging, regardless of the charger you use, the iPad Mini is capable of drawing up to about 10 watts of power. That means the charger the iPad Mini ships with by default is underpowered. Charge the iPad Mini with a full-size, iPad 4 charger and it will fill up, ready for use, in half the time.
It's also worth paying attention to the wattage of generic USB chargers if you have any. Increasingly, you're able to buy power adaptors and multiblocks with USB ports built in. Many of these are underpowered, at 2.5 watts. Plug your iPad, iPhone or iPad Mini into one of these and you'll notice that it'll take your device quite a bit longer to charge.