Centralized radio access network (C-RAN) has been considered as an architectural solution able to reduce capital and operational expenditure in dense 5G cellular networks while allowing better network performance. The C-RAN approach decouples baseband units from antenna sites and places them in selected locations, connected by the so-called fronthaul links. These links require expensive high-capacity connections, thus calling for cost-efficient deployment. This paper presents a hybrid fronthaul solution for C-RAN based on both optical fibers and free-space optics (FSO) to enhance fronthaul flexibility and minimize deployment costs. Two design strategies based on integer linear programming are proposed for both greenfield and brownfield deployments. The first strategy is referred to as joint planning (JP) and is based on the joint minimization of the number of deployed remote radio heads (RRHs) and the cost of the hybrid fiber/FSO fronthaul. The second strategy is based on two-step disjoint planning (DP) that first identifies a cost-optimal RRH placement and then finds the corresponding minimum cost deployment for the fronthaul links. Results obtained with JP and DP are compared in dense urban area scenarios (i.e., with characteristics similar to festivals or concerts), highlighting the advantage of the JP approach compared to DP, both in terms of costs and an enhanced flexibility during the network design process.