function y = PHI( x )
%PHI Summary of this function goes here
%   Detailed explanation goes here
b=0.12;
a=0.22;
c=0.32;

if x<=b
    y1 = 0;
    x1 = 0;
    y2 = a;
    x2 = b;
else if x>b & x<=c
    y1 = a;
    x1 = b;
    y2 = a;
    x2 = c;
    else
    y1 = a;
    x1 = c;
    y2 = 0;
    x2 = 1;
    end
end
k = (y2-y1)/(x2-x1);
b = -(x1*y2 - x2*y1)/(x2-x1);
y = k*x + b;