• Quick start guide
  • Connections

  • Connections
  • 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. It is not
    possible to open a TCP/IP connection using the hostname
    localhost you must use instead.

    Example #1 Special meaning of localhost

    = new mysqli("localhost""user""password""database");
    if (
    $mysqli->connect_errno) {
    "Failed to connect to MySQL: (" $mysqli->connect_errno ") " $mysqli->connect_error;
    $mysqli->host_info "\n";

    $mysqli = new mysqli("""user""password""database"3306);
    if (
    $mysqli->connect_errno) {
    "Failed to connect to MySQL: (" $mysqli->connect_errno ") " $mysqli->connect_error;

    echo $mysqli->host_info "\n";

    The above example will output:

    Localhost via UNIX socket via TCP/IP

    Connection parameter

    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


    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

    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

    The mysqlnd library and the MySQL Client Library
    (libmysqlclient) implement the same logic for determining

    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(), 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

    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 a unused persistent connection for a given
    combination of host, username, password, socket, port and default
    database can not 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() function is an expensive
    operation. For best performance, users may want to recompile the
    extension with the compile flag
    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