  The isovalently doped high-$T_{\text{c}}$ superconductor
BaFe$_2$(As$_{1-x}$P$_x$)$_2$ is characterized by a rich temperature-doping
phase diagram, which includes structural, antiferromagnetic, and
superconducting phase transitions. In this work, we use photomodulated
reflectance at 1.5 eV to detect the onset of linear birefringence in the normal
state, and by inference, the breaking of four-fold rotational (C$_4$) symmetry.
We find that C$_4$ breaking takes place in the normal state over a range of $x$
that spans the optimal P concentration, unlike the line of
orthorhombic-tetragonal and magnetic phase transitions, which terminates at, or
just below, the optimal value of $x$.
