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PHP Data Objects

Concept: Data Access Abstraction


1/  Be light-weight
2/  Provide common API for common database operations
3/  Be performant
4/  Keep majority of PHP specific stuff in the PDO core (such as persistent
    resource management); drivers should only have to worry about getting the
    data and not about PHP internals.

Transactions and autocommit

When you create a database handle, you *should* specify the autocommit
behaviour that you require.  PDO will default to autocommit on.

$dbh = new PDO("...", $user, $pass, array(PDO_ATTR_AUTOCOMMIT => true));

When auto-commit is on, the driver will implicitly commit each query as it is
executed.  This works fine for most simple tasks but can be significantly
slower when you are making a large number of udpates.

$dbh = new PDO("...", $user, $pass, array(PDO_ATTR_AUTOCOMMIT => false));

When auto-commit is off, you must then use $dbh->beginTransaction() to
initiate a transaction.  When your work is done, you then call $dbh->commit()
or $dbh->rollBack() to persist or abort your changes respectively.  Not all
databases support transactions.

You can change the auto-commit mode at run-time:

$dbh->setAttribute(PDO_ATTR_AUTOCOMMIT, false);

Regardless of the error handling mode set on the database handle, if the
autocommit mode cannot be changed, an exception will be thrown.

Some drivers will allow you to temporarily disable autocommit if you call
$dbh->beginTransaction().  When you commit() or rollBack() such a transaction,
the handle will switch back to autocommit mode again.  If the mode could not
be changed, an exception will be raised, as noted above.

When the database handle is closed or destroyed (or at request end for
persistent handles), the driver will implicitly rollBack().  It is your
responsibility to call commit() when you are done making changes and
autocommit is turned off.


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