What is the structural formula for PH_3?

In the case of ${\text{PH}}_{3}$, you know that hydrogen only has one electron, and so it can only make a maximum of one single bond (most of the time).
Therefore, phosphorus ($\text{P}$) must be the central atom, and you just draw hydrogen atoms around it. Phosphorus is right below nitrogen, so it has the same number of valence electrons as nitrogen.
As a result, ${\text{PH}}_{3}$ (phosphorus trihydride) and ${\text{NH}}_{3}$ (ammonia) are isoelectronic (same number of electrons, or similar electronic structure), and they have the same molecular geometry as well: