The behaviour of these functions is affected by settings in php.ini.
INI_SYSTEM
INI_ALL
Here's a short explanation of the configuration directives.
xmysqlnd.collect_memory_statistics
xmysqlnd.collect_statistics
xmysqlnd.debug
xmysqlnd.mempool_default_size
xmysqlnd.net_read_timeout
xmysqlnd.trace_alloc