Php -- PDO-connection and connection management

The PDO of the PHP database abstraction layer-connections and connection management connections are established by creating a PDO base class instance. No matter which driver is used, the PDO class name is used. The constructor receives parameters used to specify the database source (the so-called DSN) and possibly including the user name and password (if any.

Example #1 connect to MySQL

If any connection error occurs, a PDOException object is thrown. If you want to handle the error status, you can capture the exception or leave it to the global exception handling program of the application set through set_exception_handler.

Example #2 handle connection errors

 query('SELECT * from FOO') as $row) {            print_r($row);        }        $dbh = null;    } catch (PDOException $e) {        print "Error!: " . $e->getMessage() . "
";        die();    }?>

If the application does not capture exceptions in the PDO constructor, the default action taken by the zend Engine is to end the script and display a tracing trace. This tracing trace may leak complete database connection details, including the user name and password. Therefore, it is the responsibility to catch exceptions explicitly (via catch statements) or implicitly (via set_exception_handler.

After the data is successfully connected, a PDO class instance is returned to the script. this connection remains active during the lifecycle of the PDO object. To close the connection, you need to destroy the object to ensure that all references to the remaining object are deleted. you can assign a NULL value to the object variable. If you do not explicitly do this, PHP will automatically close the connection when the script ends.

Example #3 close a connection

Many web applications benefit from using persistent connections to database services. Persistent connections are not closed and cached after the script ends. they are reused when another script with the same creden。 connects to the request. Persistent connection cache can avoid the overhead of creating a new connection between each script and the database, so that the web application can be faster.

Example #4 persistent connection

  true    ));?>

To use persistent connections, you must set PDO: ATTR_PERSISTENT in the driver option array passed to the PDO constructor. If you set this property with PDO: setAttribute () after object initialization, the driver will not use persistent connections.

If you use the pdo odbc driver and the ODBC Library supports the ODBC Connection pool (either unixODBC or Windows; there may be more), we recommend that you do not use persistent PDO connections, instead, the connection cache is left to the ODBC Connection pool layer for processing. The ODBC Connection pool is shared with other modules in the process. if the PDO cache connection is required, the connection will never be returned to the ODBC Connection pool, resulting in the creation of additional connections to serve other modules.