Connections
The MySQL server supports the use of different transport layers for
connections. Connections use TCP/IP, Unix domain sockets or Windows named
pipes.

The hostname localhost has a special meaning. It is bound to the use of Unix
domain sockets. To open a TCP/IP connection to the localhost, 127.0.0.1 must
be used instead of the hostname localhost.



	Example #1 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 #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 localhost. If socket is unset or empty, and a
Unix socket connection is requested, then a connection to the default socket
on /tmp/mysql.sock is attempted.

On Windows systems, the host name . is 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 \\.\pipe\MySQL.

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 3306.

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 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 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
MYSQLI_NO_CHANGE_USER_ON_PCONNECT being set.

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

mysqli::__construct()
mysqli_init()
mysqli::options()
mysqli::real_connect()
mysqli::change_user()
$mysqli::host_info
MySQLi Configuration Options
Persistent Database Connections




User Contributed Notes 1 note



xylene2301 at gmail dot com 1 year ago
	"The hostname localhost has a special meaning. It is bound to the use of Unix domain sockets. To open a TCP/IP connection to the localhost, 127.0.0.1 must be used instead of the hostname localhost."

Is there something I missed or is this 1st provision incorrect?
On windows11, using PHP 7.4 and MySQL 8,

$conn = new mysqli("localhost", "root", "pword", "tstDB");
echo $conn->host_info . "\n";

returns:
localhost via TCP/IP Connected to the database.