Question: Quoting proc name works in plot but not integration

Why is it that the command

plot( 'dphidx'(x,y,10,5,1/10), x= 0..1, y=0..1)

gives a good plot but I get an error for

evalf(Int(Int( 'dphidx'(x,y,10,5,1/10), x= 0..1) y=0..1))

Error, (in dphidx) cannot determine if this expression is true or false: 99/10 < x and x < 10.

Here's the code for dphidx:

dphidx:=proc(x,y,xj,yj,h) local v; if xj - h<x and x<xj then if yj - h<y and y<yj+h then if y<=yj then if yj - y<xj - x then v:=1/h else v:=0 end if else if y - yj<x - xj+h then v:=1/h else v:=0 end if end if else v:=0 end if elif xj<=x and x<xj+h then if yj - h<y and y<yj+h then if yj<=y then if y - yj<x - xj then v:=-1/h else v:=0 end if else if x - xj<y - yj+h then v:=-1/h else v:=0 end if end if else v:=0 end if else v:=0 end if end proc:

I am using Maple 13.  Thanks.

Thanks.