Thaw phyllo and unroll. Put 13 sheets on work surface and cover rest with very slightly dampened cloth. Using very sharp knife, cut phyllo into 20 (3 inch) squares, discarding 1 inch trim from short side. Fold four corners of each square to meet in center, making envelope. Set squares in 17- by 12 inch jellyroll pan. Repeat with remaining 13 sheets phyllo.
Divide pistachios among phyllo pockets. Heat butter in small saucepan until hot to the touch. Pour butter into pan, making sure it gets to every part. Bake at 400 degrees until done, about 20 minutes.
Dissolve sugar in water. Add lemon juice and boil 2 minutes.
Remove phyllo from oven. Tip pan to drain off excess butter. Return tray to oven 1 minute, then remove from oven. Just before serving, pour boiling syrup into tray.