Catalyzes the elongation by iterative transfer of p-hydroxybenzoyl group from FadD22 (pHBA-S-FAdD22) to form p-hydroxyphenylalkanoate (pHPA) intermediates during phenolphthiocerol (PPOL) biosynthesis. PPOL is an important intermediate in the biosynthesis of phenolic glycolipid (mycosid B). a fatty acyl-[ACP] + H(+) + malonyl-[ACP] = a 3-oxoacyl-[ACP] + CO2 + holo-[ACP] Binds 1 phosphopantetheine covalently. Lipid metabolism; fatty acid biosynthesis. Disruption of pks15/1 abolishes the production of phenolphthiocerol. Belongs to the thiolase-like superfamily. Beta-ketoacyl-ACP synthases family.