The first part of this paper is devoted to a wavenumber-explicit stability analysis of a planar Helmholtz problem with a perfectly matched layer. We prove that, for a model scattering problem, the H1 norm of the solution is bounded by the right-hand side, uniformly in the wavenumber k in the highly oscillatory regime. The second part proposes two numerical discretizations: an hp finite element method and a multiscale method based on local subspace correction. The stability result is used to relate the choice of parameters in the numerical methods to the wavenumber. A priori error estimates are shown and their sharpness is assessed in numerical experiments.