ConnectionsThe MySQL server supports the use of different transport layers for connections. Connections use TCP/IP, Unix domain sockets or Windows named pipes.
The hostname
Example #1 Special meaning of localhost
The above example will output: Localhost via UNIX socket 127.0.0.1 via TCP/IP Connection parameter defaults Depending on the connection function used, assorted parameters can be omitted. If a parameter is not provided, then the extension attempts to use the default values that are set in the PHP configuration file.
Example #2 Setting defaults mysqli.default_host=192.168.2.27 mysqli.default_user=root mysqli.default_pw="" mysqli.default_port=3306 mysqli.default_socket=/tmp/mysql.sock The resulting parameter values are then passed to the client library that is used by the extension. If the client library detects empty or unset parameters, then it may default to the library built-in values. Built-in connection library defaults
If the host value is unset or empty, then the client library will
default to a Unix socket connection on
On Windows systems, the host name
If neither a Unix domain socket based not a Windows named pipe based connection
is to be established and the port parameter value is unset, the library
will default to port The mysqlnd library and the MySQL Client Library (libmysqlclient) implement the same logic for determining defaults. Connection options Connection options are available to, for example, set init commands which are executed upon connect, or for requesting use of a certain charset. Connection options must be set before a network connection is established. For setting a connection option, the connect operation has to be performed in three steps: creating a connection handle with mysqli_init or mysqli::__construct, setting the requested options using mysqli::options, and establishing the network connection with mysqli::real_connect. Connection pooling The mysqli extension supports persistent database connections, which are a special kind of pooled connections. By default, every database connection opened by a script is either explicitly closed by the user during runtime or released automatically at the end of the script. A persistent connection is not. Instead it is put into a pool for later reuse, if a connection to the same server using the same username, password, socket, port and default database is opened. Reuse saves connection overhead. Every PHP process is using its own mysqli connection pool. Depending on the web server deployment model, a PHP process may serve one or multiple requests. Therefore, a pooled connection may be used by one or more scripts subsequently. Persistent connection If an unused persistent connection for a given combination of host, username, password, socket, port and default database cannot be found in the connection pool, then mysqli opens a new connection. The use of persistent connections can be enabled and disabled using the PHP directive mysqli.allow_persistent. The total number of connections opened by a script can be limited with mysqli.max_links. The maximum number of persistent connections per PHP process can be restricted with mysqli.max_persistent. Please note that the web server may spawn many PHP processes.
A common complain about persistent connections is that their state is
not reset before reuse. For example, open and unfinished transactions are not
automatically rolled back. But also, authorization changes which happened
in the time between putting the connection into the pool and reusing it
are not reflected. This may be seen as an unwanted side-effect. On the contrary,
the name The mysqli extension supports both interpretations of a persistent connection: state persisted, and state reset before reuse. The default is reset. Before a persistent connection is reused, the mysqli extension implicitly calls mysqli::change_user to reset the state. The persistent connection appears to the user as if it was just opened. No artifacts from previous usages are visible.
The mysqli::change_user call is an expensive operation.
For best performance, users may want to recompile the extension with the
compile flag It is left to the user to choose between safe behavior and best performance. Both are valid optimization goals. For ease of use, the safe behavior has been made the default at the expense of maximum performance. See also
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