Driver function
$ R = 30 ; echo " Focal ▁ length ▁ of ▁ spherical " , " concave ▁ mirror ▁ is ▁ : ▁ " , focal_length_concave ( $ R ) , " ▁ units STRNEWLINE " ; echo " Focal ▁ length ▁ of ▁ spherical " , " ▁ convex ▁ mirror ▁ is ▁ : ▁ " , focal_length_convex ( $ R ) , " ▁ units " ; ? >