PHP会话不在docker上工作

Question

I am trying to set up a PHP application but something seems to be wrong with session handling.

As you can see the configuration is quite standard. I checked the /tmp permissions from the container and got

root@2217e6d458ad:/# ls -ld /tmp
drwxrwxrwt 1 root root 4096 Dec 28 08:03 /tmp

I also checked in runtime following snippet:

$file = fopen('/tmp/foo.log', 'w');
fwrite($file, 'test'.PHP_EOL);
fclose($file);
echo 'path: '. session_save_path().'<br>';
var_dump(is_writable(session_save_path()));

Now, the file /tmp/foo.log was created, the path was empty but the var_dump results with bool(false).

What else should I check??