If a user opens the same php-page concurrently twice, in the same browser window. Will both php scripts be executed fully to the end?
Yes. Since the person cannot possibly open them both simultaneously (and if they did the requests would still reach the server sequentially) the script will run fully for one page request and then run fully again for the other (possibly with some additional executions for requests by other people in between).
Many thanks, for at fine reply!