A siphon is depicted in the figure. The liquid present in the setup...

Points A and B are located on the free surfaces of the liquid in the upper and lower containers, respectively.

Since both containers are open to the atmosphere, the pressure at the free surface of the liquid in each container is equal to the atmospheric pressure ($P_{\text{atm}}$).

Therefore, $P_A = P_{\text{atm}}$ and $P_B = P_{\text{atm}}$.

The pressure difference between points A and B is:

$P_B - P_A = P_{\text{atm}} - P_{\text{atm}} = 0$