Phobias can be described in classical conditioning terms as stimulus generalisation that has gone way too far.
Say, for instance, that you like dogs. They don’t make you afraid at all. They are neutral stimuli. One day, however, you are bitten by a German shepherd (this happened to me once) who comes running out of nowhere and sinks his teeth with unseemly violence deep into your leg. The bite is an unconditioned stimulus that causes an unconditioned startle response, fear and pain. You don’t need to learn to react with fear to that kind of stimulus.
You might after this single trial become a little nervous of meeting that dog again. He is no longer a neutral stimulus. He has become a conditioned stimulus that makes you fearful any time you see him. Your fear is now elicited without the bite. The fear has become a conditioned response.
If you then become afraid of other German shepherd dogs, you are displaying stimulus generalisation. If you go even further and become anxious when you meet Great Danes, collies, spaniels and even chihuahuas, then your stimulus generalisation is turning into a full-fledged phobia.
A phobia like this is reinforced by the absence of fear when you avoid dogs by crossing the street when you see one. Whew, you think. Your avoidance reduces your fear, which means the avoidance behaviour is negatively reinforced. So the phobia might have begun through a form of classical conditioning, but it is maintained through operant conditioning.
The video below shows a method of using the principles of classical conditioning to treat fear-related anxiety. When people develop an intense phobic fear, it can impair their lives. That’s why finding ways to overcome intense fears and phobias is an important aim of psychotherapists.