  Implicit and explicit raytracing-photoionisation algorithms have been
implemented in the author's radiation-magnetohydrodynamics code. The algorithms
are described briefly and their efficiency and parallel scaling are
investigated. The implicit algorithm is more efficient for calculations where
ionisation fronts have very supersonic velocities, and the explicit algorithm
is favoured in the opposite limit because of its better parallel scaling. The
implicit method is used to investigate the effects of initially uniform
magnetic fields on the formation and evolution of dense pillars and cometary
globules at the boundaries of HII regions. It is shown that for weak and medium
field strengths an initially perpendicular field is swept into alignment with
the pillar during its dynamical evolution, matching magnetic field observations
of the`Pillars of Creation' in M16. A strong perpendicular magnetic field
remains in its initial configuration and also confines the photoevaporation
flow into a bar-shaped, dense, ionised ribbon which partially shields the
ionisation front.
