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, $pass);
foreach($dbh->query('SELECT * from FOO') as $row) {
print_r($row);
}
$dbh = null;
} catch (PDOException $e) {
print "Error!: " . $e->getMessage() . "<br/>";
die();
}
?>
Warning
If your application does not catch the exception thrown from the PDO
constructor, the default action taken by the zend engine is to terminate
the script and display a back trace. This back trace will likely reveal
the full database connection details, including the username and
password. It is your responsibility to catch this exception, either
explicitly (via a catch statement) or implicitly via
set_exception_handler().
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.
Example #3 Closing a connection
<?php
$dbh = new PDO('mysql:host=localhost;dbname=test', $user, $pass);
// use the connection here
// and now we're done; close it
$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
));
?>
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.
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.
Load balance : Server 03
