Q.In what way can it be proved that PH3 is basic in nature?

Phosphine, $PH_3$, is a pyramidal, colourless, poisonous gas prepared by heating white phosphorus with concentrated NaOH in an inert atmosphere, or by treating calcium phosphide with water. Unlike $NH_3$, it is only feebly basic because phosphorus is larger and less electronegative than nitrogen, so its lone pair is more diffuse and less available for donation; nonetheless $PH_3$ reacts with strong acids like HI to form phosphonium salts, $PH_3 + HI \rightarrow PH_4I$, and this salt-forming reaction is itself the standard demonstration of its (weak) basic character, exactly analogous to $NH_4^+$ formation from $NH_3$. Protonation of $PH_3$ to $PH_4^+$ increases the bond angle from about $93.5°$ in $PH_3$ to close to the tetrahedral $109.5°$ in $PH_4^+$, because the lone pair (which compresses the H-P-H angle through greater lone-pair-bond-pair repulsion) is replaced …