  Recently, the planar Hall effect has attracted tremendous interest. In
particular, an in-plane magnetization can induce an anomalous planar Hall
effect with a $2\pi/3$ period for hexagon-warped energy bands. This effect is
similar to the anomalous Hall effect resulting from an out-of-plane
magnetization. However, this anomalous planar Hall effect is absent in the
planar Hall experiments. Here, we explain its absence, by performing a
calculation that includes not only the Berry curvature mechanism as those in
the previous theories, but also the disorder contributions. The conventional
$\pi$-period planar Hall effect will occur if the mirror reflection symmetry is
broken, which buries the anomalous one. We show that an in-plane strain can
enhance the anomalous Hall conductivity and changes the period from $2\pi/3$ to
$2\pi$. We propose a scheme to extract the hidden anomalous planar Hall
conductivity from the experimental data. Our work will be helpful in detecting
the anomalous planar Hall effect and could be generalized to understand
mechanisms of the planar Hall effects in a wide range of materials.
