Answer
The phase difference increases.
Work Step by Step
When point $P$ is very far away, the path length from $S_1$ to point $P$ and the path length from $S_2$ to point $P$ are essentially the same distance.
Therefore, the path length difference is approximately zero.
In this case, the phase difference is zero.
As $P$ moves along the x axis toward $S_1$, the path length difference starts to increase, when we consider the path length from $S_1$ to point $P$ and the path length from $S_2$ to point $P$.
Therefore, the phase difference increases.
