Hydrogen-rich compounds have been extensively studied both theoretically and experimentally in the quest for novel high-temperature superconductors. Reports on sulfur hydride attaining metallicity under pressure and exhibiting superconductivity at temperatures as high as 200 K have spurred an intense search for room-temperature superconductors in hydride materials. Recently, compressed phosphine was reported to metallize at pressures above 45 GPa, reaching a superconducting transition temperature (T-C) of 100 K at 200 GPa. However, neither the exact composition nor the crystal structure of the superconducting phase have been conclusively determined. In this work, the phase diagram of PHn (n = 1,2,3,4,5,6) was extensively explored by means of ab initio crystal structure predictions using the minima hopping method (MHM). The results do not support the existence of thermodynamically stable PHn compounds, which exhibit a tendency for elemental decomposition at high pressure even when vibrational contributions to the free energies are taken into account. Although the lowest energy phases of PH1,2,3 display T-C`s comparable to experiments, it remains uncertain if the measured values of T-C can be fully attributed to a phase-pure compound of PHn.