I did make other experience with this function:
swf_startshape ($this->gButton);
swf_addcolor(0,0,0,0);
swf_shapelinesolid(1,0,0,0,5);
swf_shapefillsolid($r,$g,$b,1);
swf_shapemoveto(0,10);
swf_shapelineto(60,10);
swf_shapelineto(60,-5);
swf_shapelineto(-35,-5);
swf_shapelineto(-35,10);
swf_endshape($this->gButton);
Here, swf_shapelineto doesnt set the given values to the
current position. In case this would not build a rectangle -
but it does.