How are persistent database connections stored in PHP?
sqlite_popen uses the (executor) global hashtable "persistent_list" to store the connection resource. This hashtable is not part of the php "instance" which executes your script but of the php "runtime", which means it's not cleared/removed after the script is done but held in memory as long as the php .so/.dll/fastcgi.exe.
You cannot access EG(persistent_list) from within a php script but any php module/extension can.