None of the languages you mentioned in your OP has anything to do with the internet or creating web apps so take that stuff and file it away some where. They will just get in your way. Though with that said you will see similarities between the various languages you know and the ones you want to learn, the problem that may come up is that you know how to code THAT way which is NOT the same as what you want to be able to do. Some relearning may come into play, especially if you have bad habits from the past.
PHP is also fully OOP so if you do not know how to code in OOP then you will be learning that too.
For PHP the defacto current version is 7.1 to 7.2 though 7.4 is out now you won’t be really using that in real every day yet. If you learn PHP 5 there really isn’t much difference between that and 7, just use the manual which you can download, and look at the difference in functions you want to use. Most that work in 5 work in 7 though there are some that only exist for 7 and replace older functions in PHP 5. Mainly specific to password generation and retrieval though most everything else works either the same or similar with minor differences.
I hope these posts I shared help. I don’t have anything more I can add.