The actual way to check it is to switch to PHP 8.4 and check the error log unless your site goes down. But don't do that. Create a staging site and check there. However, I don't recommend switching to 8.4 yet. There are many plugins still catching up with 8.3, which is not yet outdated and will still be supported, so the tops I recommend is 8.3. Also, when changing PHP versions it's important that you create a backup and switch version by version with some time between changing versions, until you get to the desired one. WP claims minimum supported PHP version threshold (7.4, though many plugins are still declaring 5.6). WP itself can't check if your plugin is compatible or not - and I don't count errors and warnings in regards to this. WP simply reads the text from the readme and style.css files from themes and plugins and that's it, and that's the info developers add on their own. To summarize: Don't switch to 8.4 yet, not needed. 8.3 is completely fine and will be fine and will have support until 2027. Create a staging site or copy your website elsewhere (localhost), change versions and check. Create a backup Change version by version, with some time between changes. I would also add to disable plugins when doing so, and only after enable one by one (after each change). Test all options thoroughly, inspect the error log for any errors. If you see warnings related to some plugins, inform plugin developers for warnings (these are related to future PHP updates and will not crash your website). If the outcome is good, update your live site Hope this helps and best of luck More on reddit.com

If you use a tool, I'd still encourage setting up a staging instance if you don't already have one and making the PHP upgrade to that location first. Compatibility checkers aren't foolproof so even if a checker says you're good to go, you're still playing with fire if you push it through on production based on a scan alone, in my opinion. More on reddit.com

Well it's "Performance" tag, not security, so it's not wrong. More on reddit.com

PHP versions below 8.1 are no longer being supported ( https://php.watch/versions ) i.e. if a bug or vulnerability is discovered in 7.4 or 8, it ain't getting fixed. There's not really any reason to NOT upgrade. Recent versions offer performance improvements. You should always be running up to date WP, theme and plugins anyway - if you're running a plugin or theme that requires php 7.4, it's just a matter of time until you're hacked, because the theme/plugin isn't getting patched. More on reddit.com