We report a broadband photonic-based microwave hybrid combiner (PMHC) using a dual-polarization dual-parallel Mach-Zehnder modulator. The broadband PMHC with arbitrarily tunable phase shift and power combining ratio can overcome several limitations of conventional microwave combiners in terms of tunability and bandwidth. By simply adjusting the biases of the modulator, the phase shift and power ratio of the two combined microwave signals are independently, continuously, and freely tunable. The proposed PMHC is theoretically analyzed and experimentally verified. In a proof-of-concept experiment, two sinusoidal microwave waveforms are successfully combined to form rectangular and triangular waveforms, respectively.