Upgrading to PHP 8.4 blocked rendering Phorge in my browsers (see https://we.phorge.it/D25887#26909) and using arc diff (see https://we.phorge.it/D25887#26924) due to plain text deprecation warnings prefixing output in a blob expected to be valid JSON.

After fixing that by applying the two patches in T15989, PHP deprecation warnings still blocked using arc diff due to ending up in STDERR. I don't think that's expected: