I know that hot water makes dissolving injection mixtures easier, but I noticed this on the Stella Culinary Brining video.
The whole video is very educational, but the part relevant to phosphates is at about 7:20. Apparently if this is correct boiling or overheating phosphates pretty much renders them useless. Thats not to say that the injection is completely useless. It won't affect flavor. Its the moisture retention properties of the phosphates that are "killed".
And hey this is just one source, but I figured I would pass it along for what its worth.