How to know which atom is the central atom in VSEPR theory?
You know that phosphorus has 5 valence electrons and that chlorine has 7 valence electrons ... and that you somehow have to combine them.

As chlorine is more electronegative (Allred-Rochow: Cl 2.83, P 2.06) you know that each chlorine will attract one electron from the phosphorus to get the noble gas configuration with 8 electrons in the valence shell.

Now that in turn means that there will be one single bond between the phosphorus and each chlorine, which automatically makes phosphorus the center.