We propose the concept and design of terahertz (THz) phase shifters for phased antenna arrays based on integrally-gated graphene parallel-plate waveguides (GPPWGs). We show that an active transmission-line may be realized by combining GPPWGs with double-gate electrodes, in which the applied gate voltage can control the guiding properties of the gated sections. This may enable the realization of THz electronic switches and tunable loaded-lines for sub mm-wave antenna systems. Based on these active components, we theoretically and numerically demonstrate several digital and analog phase shifter designs for THz frequencies, with a wide range of phase shifts and small return loss, insertion loss and phase error. The proposed graphene-based phase shifters show significant advantages over other available technology in this frequency range, as they combine the low-loss and compact-size features of GPPWGs with electrically-programmable phase tuning. We envision that these electronic phase shifters may pave the way to viable phased-arrays and beamforming networks for THz communications systems, as well as for high-speed, low-RC-delay, inter/intra-chip communications.