Search notes:

/run/user/$uid

/run/user/$uid is created by pam_systemd and used to store non-essential runtime files or objects, such as sockets, named pipes, keyring daemon files, pulseaudio files etc.
This directory is owned by the user whose uid is $uid.
The permission bits for this directory need to be 0700.
The directory is created when the user (with $uid) logs in and is destroyed when he logs out again.
The environment variable $XDG_RUNTIME_DIR might(?) point to this directory (compare XDG)

See also

Prior to systemd, the files that are now stored in /run/user/$uid were typically stored in /tmp.
Using /home/_username_ was not practical because this directory was often mounted over network filesystems.
/run

Index

Fatal error: Uncaught PDOException: SQLSTATE[HY000]: General error: 8 attempt to write a readonly database in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php:78 Stack trace: #0 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(78): PDOStatement->execute(Array) #1 /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php(30): insert_webrequest_('/notes/Linux/fh...', 1788560582, '216.73.216.30', 'Mozilla/5.0 App...', NULL) #2 /home/httpd/vhosts/renenyffenegger.ch/httpsdocs/notes/Linux/fhs/run/user/uid/index(54): insert_webrequest() #3 {main} thrown in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php on line 78