Solubility of lead phosphate
The precipitation of lead phosphate would be due to the following reaction.

$$\ce{3Pb^{2+} + 2PO4^{3-} &lt;=&gt; Pb3(PO4)2}$$ 

In acidic solution the phosphate ions would be protonated, thus reducing the concentration of $\ce{PO4^{3-}}$ and increasing the solubility of $\ce{Pb^{2+}}$.

But the precipitation of the phosphate is not lead's only reaction in alkaline solution.  Lead can also form a hydroxide precipitate. In strongly alkaline solution lead can also form various soluble hydroxide complexes which would reduce the concentration of $\ce{Pb^{2+}}$ thus increasing the solubility of $\ce{Pb3(PO4)2}$.