PHP Algorithm for Smooth and interpolate GPS coordinates

I'm using a function in php where i send 3 (x,y) points and i get and array with interpolated middle points smoothed The functions it's this: [login to view URL] The code works pretty fine with pixels (x,y), but with GPS coordinates i get slightly different path and not as smooth at all (red line in the image), also the coordinates generated are not equidistant ( black dots) that is essential You can improve the current function or made one from scratch, in both cases i want $tension (smooth tension) and $steps (numbers of interpolations per section) features available and the interpolations MUST BE equidistant The reference points to test are: (x,y) 11.04 , 6.39 8.40 , 8.54 8.75 , 11.96 (lat,long) 37.893754,-0.960408 37.893701, -0.960499 37.893608,-0.960492