consider the following molecules:SiH4, PH3, H2S. What is similar about these molecules? What is different about these molecules and why?
All contain H atoms.
The difference is that H has an oxidation state of +1 in H2S (the usual number for hydrogen but it is -1 in SiH4 and PH3. You can find some sources that give H a -1 in PH3 and some that give it a +1 in PH3. Since this question is stated as it is I assume the answer is to give it a -1 oxidation state. If you look up the electronegativities you will find 2.20 for H and 1.82 for P which makes P the + and H the -; therefore, I agree with those who assign +3 for P and -1 each for H in PH3.