Type III polyketide synthase that catalyzes the synthesis of phloroglucinol from three molecules of malonyl-CoA (PubMed:15826166, PubMed:16931521). In addition to its ability to produce phloroglucinol from malonyl-CoA, it exhibits broad substrate specificity, accepting C4-C12 aliphatic acyl-CoAs and phenylacetyl-CoA as the starters to form C6-polyoxoalkylated alpha-pyrones from sequential condensation with malonyl-CoA (PubMed:16931521). 3 H(+) + 3 malonyl-CoA = 1,3,5-trihydroxybenzene + 3 CO2 + 3 CoA kcat is 10 min(-1) with malonyl-CoA as substrate (PubMed:15826166). kcat is 24 min(-1) with malonyl-CoA as substrate (PubMed:16931521). kcat is 7.3 min(-1) with malonyl-CoA as substrate (PubMed:18437267). Antibiotic biosynthesis. Phloroglucinol is widely used in industry and in the synthesis of pharmaceuticals. PhlD can be used to produce this chemical, however it exhibits low productivity, which is a bottleneck for large-scale application. Different stategies have been developed to improve the stability and the productivity of the enzyme. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.