The Russian Scientist, Pavlov, demonstrated “conditioning” of dogs in an experiment involving exposure of the dogs to meat powder (the unconditional stimulus, or UCS), resulting in salivation (the conditional response, or CR).  He then exposed the dogs to UCS plus a ringing bell (conditional stimulus, or CS), each and every time they were given the meat powder, they became conditioned to salivate (CR) in response to exposure to the CS (ringing bell) alone.  The events could be summarized as follows:

Begining situation:  UCS = CR

Conditioning period:  CS + UCS = CR

Dogs after conditioning:  CS = CR

Conditioning can indeed lead to a phobia.  A phobia is an unreasonable fear and avoidance of something that is disproportionate to the actual danger involved.

If dogs are given an electrical shock every time they enter one of three cages, they will develop a phobia of entering. They will avoid entering the cage in question.  In this case the Electrical shock is the UCS, and it causes the CR (in this case, a yelp and pain).  The CS is the entry into a cage.  By combining CS and UCS for the cage in question, the dog becomes conditioned to fear entering.

Some psychologists feel that acquisition of phobias requires some form of conditioning.  For example, a fear of heights could result from a traumatic event in childhood such as being stuck for some time at the top of a Ferris wheel.