Phenol is treated with chloroform and aqueous $\mathrm{NaOH}$ at $340\mathrm{K}$ and the mixture is then acidified (Reimer-Tiemann reaction). The major product is

Dichlorocarbene generated from $\mathrm{CHCl}{}_3$ and base attacks the phenoxide ring mainly at the ortho position, giving $-\mathrm{CHCl}{}_2$ which hydrolyses to $-\mathrm{CHO}$. The major product is 2-hydroxybenzaldehyde (salicylaldehyde). The acids need $\mathrm{CO}{}_2$ (Kolbe) and the phenolic OH is retained.