The behaviour of these functions is affected by settings in php.ini.
Here's a short explanation of the configuration directives.
Default timeout when connecting to databases
The secret that is used to salt session id
Prefix all keys with this string. The prefix is transparent to the developer but helps making sure that keys won't collide if multiple applications are using the same database.
Whether to allow session failover in case a server dies.
How many read/write or connection failures is allowed before server is marked as failed.
Defines the divisor for the health check probability. If there are failed servers and the probability matches, the servers are health checked and in case the server seems healthy, it will be added back to the pool.
Whether to use built in session expiration mechanism or delegate the expiration to a lua-script on the server-side.