if you want to place a text onto the edge of a circle you need to
get a point on the circle:
$pos_x=$radius*sin(deg2rad($angle));
$pos_y=sqrt($radius*$radius-$pos_x*$pos_x);
if you want the point to be in the middle of a pie piece
you need a starting and ending angle
$pos_x=$radius*sin(deg2rad($angle_end-($angle_start)/2));
$pos_y=sqrt($radius*$radius-$pos_x*$pos_x);
hope it helps