A porcupine’s quills are just extremely enlarged specialized hairs that have unusually (compared to other mammals) thick layers of keratin, which is the main structural component of hair and finger nails, among other things.  These thick layers of keratin make porcupine quills extremely rigid and good for poking things.

A Cat’s whiskers are also just specialized hairs.  With cats, these hairs are attached to nerves allowing them to work as extremely sensitive sensory receptors.  The nerves at the base of the whiskers are actually sensitive enough to be able to pick up even small air movements which vibrate the whiskers.  They are so sensitive that cats can even pick up air movements indoors, such as air flowing around furniture, which lets the cat know there is an object there, even when it is pitch dark. This is essential for cats because they are far sighted and have trouble seeing things clearly close up.

References:

• There is No Difference Between Fur and Hair
• What Happens if a Cat’s Whiskers are Damaged or Clipped
• Porcupines
• Keratin
• Hair