php-doc-en/reference/pdo/transactions.xml

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<chapter xml:id="pdo.transactions" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>Transactions and auto-commit</title>
 <para>
  Now that you're connected via PDO, you must understand how PDO
  manages transactions before you start issuing queries. If you've never
  encountered transactions before, they offer 4 major features: Atomicity,
  Consistency, Isolation and Durability (ACID). In layman's terms, any work
  carried out in a transaction, even if it is carried out in stages, is
  guaranteed to be applied to the database safely, and without interference
  from other connections, when it is committed. Transactional work can also
  be automatically undone at your request (provided you haven't already
  committed it), which makes error handling in your scripts easier.
 </para>
 <para>
  Transactions are typically implemented by "saving-up" your batch of
  changes to be applied all at once; this has the nice side effect of
  drastically improving the efficiency of those updates. In other words,
  transactions can make your scripts faster and potentially more robust
  (you still need to use them correctly to reap that benefit).
 </para>
 <para>
  Unfortunately, not every database supports transactions, so PDO needs to
  run in what is known as "auto-commit" mode when you first open the
  connection.  Auto-commit mode means that every query that you run has its
  own implicit transaction, if the database supports it, or no transaction
  if the database doesn't support transactions. If you need a transaction,
  you must use the <function>PDO::beginTransaction</function> method to
  initiate one. If the underlying driver does not support transactions, a
  PDOException will be thrown (regardless of your error handling settings:
  this is always a serious error condition). Once you are in a transaction,
  you may use <function>PDO::commit</function> or
  <function>PDO::rollBack</function> to finish it, depending on the success
  of the code you run during the transaction.
 </para>
 <warning>
  <para>
   PDO only checks for transaction capabilities on driver level. If certain
   runtime conditions mean that transactions are unavailable,
   <methodname>PDO::beginTransaction</methodname> will still return &true;
   without error if the database server accepts the request to start a
   transaction.
  </para>
  <para>
   An example of this would be trying to use transactions on MyISAM tables on
   a MySQL database.
  </para>
 </warning>
 <para>
  When the script ends or when a connection is about to be closed, if you
  have an outstanding transaction, PDO will automatically roll it back.
  This is a safety measure to help avoid inconsistency in the cases where
  the script terminates unexpectedly--if you didn't explicitly commit the
  transaction, then it is assumed that something went awry, so the rollback
  is performed for the safety of your data.
 </para>
 <warning>
  <para>
   The automatic rollback only happens if you initiate the transaction via
   <function>PDO::beginTransaction</function>. If you manually issue a
   query that begins a transaction PDO has no way of knowing about it and
   thus cannot roll it back if something bad happens.
  </para>
 </warning>
 <para>
  <example><title>Executing a batch in a transaction</title>
   <para>
    In the following sample, let's assume that we are creating a set of
    entries for a new employee, who has been assigned an ID number of 23.
    In addition to entering the basic data for that person, we also need to
    record their salary. It's pretty simple to make two separate updates,
    but by enclosing them within the
    <function>PDO::beginTransaction</function> and
    <function>PDO::commit</function> calls, we are guaranteeing that no one
    else will be able to see those changes until they are complete. If
    something goes wrong, the catch block rolls back all changes made
    since the transaction was started, and then prints out an error
    message.
   </para>
   <programlisting role="php">
<![CDATA[
<?php
try {
  $dbh = new PDO('odbc:SAMPLE', 'db2inst1', 'ibmdb2', 
      array(PDO::ATTR_PERSISTENT => true));
  echo "Connected\n";
} catch (Exception $e) {
  die("Unable to connect: " . $e->getMessage());
}

try {  
  $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);

  $dbh->beginTransaction();
  $dbh->exec("insert into staff (id, first, last) values (23, 'Joe', 'Bloggs')");
  $dbh->exec("insert into salarychange (id, amount, changedate) 
      values (23, 50000, NOW())");
  $dbh->commit();
  
} catch (Exception $e) {
  $dbh->rollBack();
  echo "Failed: " . $e->getMessage();
}
?>
]]>
   </programlisting>
  </example>
 </para>
 <para>
  You're not limited to making updates in a transaction; you can also issue
  complex queries to extract data, and possibly use that information to
  build up more updates and queries; while the transaction is active, you
  are guaranteed that no one else can make changes while you are in the
  middle of your work. For further reading on transactions, refer to the
  documentation provided by your database server.
 </para>
</chapter>

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