A mixture of C6H5CHO and HCHO is treated with NaOH, then the Cannizzaro’s reaction involves:
Oxidation of HCHO
Cross-Cannizzaro reactions involving HCHO almost always have formaldehyde oxidized to formic acid or formates. C6H5CHO+HCHO→C6H5CH2OH+HCOOH. Thus HCHO is oxidized to HCOOH.