Two coherent sources of light interfere and produce fringe patterns...

For central maximum, path difference is zero hence phase difference for coherent sources of light will be zero. And the relation between phase difference and path difference is, $∆ ϕ = \dfrac{2\pi}{\lambda} ∆ x$, where $\Deltax$ is path difference and $∆ ϕ$ is phase difference. So due to zero path difference phase difference is also zero.