Klein-Gordon Feynman Green's Function

A massive scalar field $\phi (x,t)$ of mass $m$ in quantum field theory satisfies the Klein-Gordon equation

$\big(\Box + m^2\big) \phi(x,t) = 0,$

where $\Box$ indicates the d'Alembert wave operator:

$\Box = -\dfrac{\partial^2}{\partial t^2} +\dfrac{\partial^2}{\partial x^2} + \dfrac{\partial^2}{\partial y^2} +\dfrac{\partial^2}{\partial z^2}.$

Find the Green's function of the Klein-Gordon operator in momentum space by Fourier transform. Note that $k = \big(E,\vec{k}\big)$ is a vector with four components: the energy of a particle, and its three components of spatial momentum.

Notation: $\text{abs}(\cdot)$ denotes the absolute value function.