The MySQL server supports the use of different transport layers for connections. Connections use TCP/IP, Unix domain sockets or Windows named pipes.
localhost has a special meaning.
It is bound to the use of Unix domain sockets. To open a TCP/IP
connection to the localhost,
be used instead of the hostname
Example 3.4 Special meaning of localhost
<?php $mysqli = new mysqli("localhost", "user", "password", "database"); echo $mysqli->host_info . "\n"; $mysqli = new mysqli("127.0.0.1", "user", "password", "database", 3306); echo $mysqli->host_info . "\n";
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 3.5 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
localhost. If socket is unset or empty, and a
Unix socket connection is requested, then a connection to the
default socket on
On Windows systems, the host name
interpreted by the client library as an attempt to open a
Windows named pipe based connection. In this case the socket
parameter is interpreted as the pipe name. If not given or
empty, then the socket (pipe name) defaults to
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 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
setting the requested options using
and establishing the network connection with
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.
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
persistent may be
understood as a promise that the state is persisted.
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
to reset the state. The persistent connection appears to the
user as if it was just opened. No artifacts from previous usages
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.
|MySQLi Configuration Options|
|Persistent Database Connections|