Search notes:

ORA-65040: Operation is not allowed from within a pluggable database

Find an init parameter that can be immediately modified but is modifiable for a PDB: 
select
   name,
   value
from
   v$parameter
where
   ispdb_modifiable = 'FALSE'      and
   issys_modifiable = 'IMMEDIATE'
order by
   name;
One of these parameters is background_core_dump. Assuming we're on a PDB, the following statement throws ORA-65040: Operation is not allowed from within a pluggable database: 
alter system set background_core_dump = full;

See also

Other Oracle error messages

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/developm...', 1758199476, '216.73.216.150', 'Mozilla/5.0 App...', NULL) #2 /home/httpd/vhosts/renenyffenegger.ch/httpsdocs/notes/development/databases/Oracle/errors/ORA-65040_Operation-is-not-allowed-from-within-a-pluggable-database(50): insert_webrequest() #3 {main} thrown in /home/httpd/vhosts/renenyffenegger.ch/php/web-request-database.php on line 78