Phenol is heated with $\mathrm{CHCl}{}_3$ and aqueous $\mathrm{NaOH}$, and the mixture is then acidified with $\mathrm{H}{}_3\mathrm{O}{}^{+}$. The major product is

This is the Reimer-Tiemann reaction. Dichlorocarbene attacks the phenoxide ion mainly at the ortho position, and hydrolysis of the $-\mathrm{CHCl}{}_2$ group followed by acidification gives 2-hydroxybenzaldehyde (salicylaldehyde); the para isomer is only minor.