Question

The title is pretty clear:

I have a PHP file and I want it to be executed only the first time even if 100.000 users try to access it at the same time.

Answer 1

You could do this with a lock file you write to the server, like:

if (!is_file("visited_once.txt"))
{
    //you're the one
    file_put_contents("visited_once.txt", "");
}

But maybe a database-solution would be more sophisticated as I’m not a 100% sure that the file solution works for 100.000 concurrent requests.

create your table

CREATE TABLE visited
(
    visit int NOT NULL,
    UNIQUE (visit)
);

do something like this on request

try
{
    $db->query("INSERT INTO visited VALUES (0)");
    //you're the one
}
catch (Exception $ex)
{
    //nope! Already visited
}

This uses a unique key and relies on the database consistency.