Phenol is heated with $\mathrm{CHCl}{}_3$ and aqueous $\mathrm{NaOH}$ at about 340 K, and the product is acidified with $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The main product is

This is the Reimer-Tiemann reaction: dichlorocarbene from $\mathrm{CHCl}{}_3$/$\mathrm{NaOH}$ attacks the phenoxide mainly at the ortho position. Hydrolysis of the $-\mathrm{CHCl}{}_2$ group gives $-\mathrm{CHO}$, so the main product is salicylaldehyde, 2-hydroxybenzaldehyde.