Even in water there is only a slight solubility of these acids. That means that in also in phosphate buffered saline no good solubility can be expected. The solublity can be improved if more Na2HPO4 or another basic substance is added to the PBS, because in this case the coumaric acid can form the salt which is good soluble in water. As potassium or sodium salt the coumaric acids will be easily soluble in PBS.

Thank you dear Burkhard, but what about DMSO?

If you are able to add a cosolvent, DMSO would be a good choice. One of the best solvents for coumaric acids is ethanol. If you can tolerate ethanol for your application you can it also use as cosolvent. But all organic cosolvents could precipitate the NaCl from the PBS. One should not use to much. This can be only clarified by experiment.