We employ the full-potential linearized augmented plane wave plus local orbital (FP L/APW + lo) method based on the density functional theory (DFT) in order to investigate the structural, elastic, electronic, and magnetic properties of ordered dilute ferromagnetic semiconductors Ga1-x Mn (x) P and In1-x Mn (x) P at (x = 0.25) in the zinc blende phase, using generalized gradient approximation, GGA (PBE). To our knowledge the elastic constants of these compounds have not yet been measured or calculated, hence our results serve as a first quantitative theoretical prediction for future study. Results of calculated electronic structures and magnetic properties reveal that both Ga0.75Mn0.25P and In0.75Mn0.25P have stable ferromagnetic ground state, and they are ideal half-metallic (HM) ferromagnetic at their equilibrium lattice constants. Also we show the nature of the bonding from the charge spin-densities calculations. The calculated total magnetic moments are 4.0 mu(B) per unit cell for both Ga0.75Mn0.25P and In0.75Mn0.25P, which agree with the Slater-Pauling rule quite well, and we observe that p-d hybridization reduces the local magnetic moment of Mn from its free space charge value and produces smaller local magnetic moments on the nonmagnetic Ga, In and P sites. The values of N (0)alpha and N (0)beta exchange constants confirm the magnetic nature of these compounds. From the robust half-metallicity of Ga0.75Mn0.25P and In0.75Mn0.25P as a function of lattice constant is also investigated.