The formula of a chloride of a metal M is $\text{MC}{l}_3$. The formula of phosphate of metal M will be:

In $\text{MC}{l}_3$, the valence of M is 3 and cation is  ${\text{M}}^{+3}$.

The formula of anion phosphate is  ${\text{PO}}_4^{-3}$.

According to criss-cross method,

The simplest form of chemical formula is ${\text{MPO}}_4$.