Pinging @Pablo Borbón32 as he might be the man with the answer 😀
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The 3.10 MP2 will include the necessary bug fixes to the core code that will allow for the PHP upgrade to happen. The Maintenance Packs focus on the application code and the environment code can be managed independently of those MPs. The flow would be something like first MP2 prepares the code and then perhaps a week later a rolling update to upgrade the PHP version in the environments.