The paper introduces a software-defined phased array radio (SDPAR) operating around the 5G NR frequency band of 28 GHz. The highly re-configurable phased array radio features beam shaping/steering control as well as data TX/RX function control from a single Python-based software interface. High speed phased array control and configuration functions are enabled by extensive on-chip digital circuits co-integrated with mmWave circuit blocks to achieve agile beam-steering and beam-shaping functions. Beams are formed using up to 128 independently controlled phased array elements using RF beamforming for groups of 16 elements, and IF/digital beamforming thereafter. Both TX and RX capabilities are enabled in both H and V polarizations. These functions are accessed through a software stack providing a top level application programming interface (API) for user programmability. The same API also controls the data communication functions with both TX and RX capabilities enabling USRP-GNU radio functions at around 28 GHz frequency. In addition to the hardware platform, this paper presents a software-only SDPAR emulator to enable fast software-only development cycles prior to real-world experiments using the SDPAR hardware. The SDPAR system platform is expected to significantly benefit networking research for mmWave phased array systems, especially targeted toward 5G communications.