  In this paper, we obtain a new characterization for univalent harmonic
mappings and obtain a structural formula for the associated function which
defines the analytic $\Phi$-like functions in the unit disk. The new criterion
stated in this article for the injectivity of harmonic mappings implies the
well-known results of Kas'yanyuk \cite{Kas59} and Brickman \cite{Brick73} for
analytic functions, but with a simpler proof than theirs. A number of
consequences of the characterization, and examples are also presented. Further
investigation provides a new method to construct univalent harmonic mappings
with the help of an improved distortion theorem.
