the difference between fnxtr and interp1(...'extrap')

Presently, I am traiting a spray images. I am looking for the virtual intersection point of two spray interfaces. In this case, extrapolation is chosen. However, I found two function: "fnxtr" and "interp1" with the parameter 'extrap'. Could someone please tell me the difference between these two function and which is better according to you? Thank you.