# Connections and Connection management

Connections are established by creating instances of the PDO base class. It doesn't matter which driver you want to use; you always use the PDO class name. The constructor accepts parameters for specifying the database source (known as the DSN) and optionally for the username and password (if any).

Example #1 Connecting to MySQL

`<?php
$dbh = new PDO('mysql:host=localhost;dbname=test', $user, $pass);
?>`

If there are any connection errors, a `PDOException` object will be thrown. You may catch the exception if you want to handle the error condition, or you may opt to leave it for an application global exception handler that you set up via set_exception_handler().

Example #2 Handling connection errors

`<?php
try {
$dbh = new PDO('mysql:host=localhost;dbname=test', $user, $pass);
} catch (PDOException $e) {
// attempt to retry the connection after some timeout for example
}`
Warning

Just like any other exception, PDOException could be caught either explicitly, via a `catch` statement, or implicitly via set_exception_handler(). Otherwise, the default behaviour of converting an uncaught exception to a `E_FATAL_ERROR` will occur. The fatal error will contain a backtrace that can leak connection details. As such, the php.ini option `display_errors` should be set to `0` on a production server.

Upon successful connection to the database, an instance of the PDO class is returned to your script. The connection remains active for the lifetime of that PDO object. To close the connection, you need to destroy the object by ensuring that all remaining references to it are deleted—you do this by assigning `null` to the variable that holds the object. If you don't do this explicitly, PHP will automatically close the connection when your script ends.

> Note: If there are still other references to this PDO instance (such as from a PDOStatement instance, or from other variables referencing the same PDO instance), these have to be removed also (for instance, by assigning `null` to the variable that references the PDOStatement).

Example #3 Closing a connection

`<?php
$dbh = new PDO('mysql:host=localhost;dbname=test', $user, $pass);
// use the connection here
$sth = $dbh->query('SELECT * FROM foo');
// and now we're done; close it
$sth = null;
$dbh = null;
?>`

Many web applications will benefit from making persistent connections to database servers. Persistent connections are not closed at the end of the script, but are cached and re-used when another script requests a connection using the same credentials. The persistent connection cache allows you to avoid the overhead of establishing a new connection every time a script needs to talk to a database, resulting in a faster web application.

Example #4 Persistent connections

`<?php
$dbh = new PDO('mysql:host=localhost;dbname=test', $user, $pass, array(
PDO::ATTR_PERSISTENT => true
));
?>`

The value of the `PDO::ATTR_PERSISTENT` option is converted to bool (enable/disable persistent connections), unless it is a non-numeric string, in which case it allows to use multiple persistent connection pools. This is useful if different connections use incompatible settings, for instance, different values of `PDO::MYSQL_ATTR_USE_BUFFERED_QUERY`.

> Note:

> If you wish to use persistent connections, you must set `PDO::ATTR_PERSISTENT` in the array of driver options passed to the PDO constructor. If setting this attribute with PDO::setAttribute() after instantiation of the object, the driver will not use persistent connections.

Warning

PDO does not perform any cleanup of persistent connections. Temporary tables, locks, transactions and other stateful changes may remain from previous usage of the connection, causing unexpected problems. See the Persistent Database Connections section for more information.

> Note:

> If you're using the PDO ODBC driver and your ODBC libraries support ODBC Connection Pooling (unixODBC and Windows are two that do; there may be more), then it's recommended that you don't use persistent PDO connections, and instead leave the connection caching to the ODBC Connection Pooling layer. The ODBC Connection Pool is shared with other modules in the process; if PDO is told to cache the connection, then that connection would never be returned to the ODBC connection pool, resulting in additional connections being created to service those other modules.

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