The export attribute is used for telling Doctrine what it should export when exporting classes to your database for creating your tables.
If you don't want to export anything when exporting you can use:
// bootstrap.php
// ...
$manager->setAttribute(Doctrine_Core::ATTR_EXPORT, Doctrine_Core::EXPORT_NONE);
For exporting tables only (but not constraints) you can use on of the following:
// bootstrap.php
// ...
$manager->setAttribute(Doctrine_Core::ATTR_EXPORT, Doctrine_Core::EXPORT_TABLES);
You can also use the following syntax as it is the same as the above:
// bootstrap.php
// ...
$manager->setAttribute(Doctrine_Core::ATTR_EXPORT,
Doctrine_Core::EXPORT_ALL ^ Doctrine_Core::EXPORT_CONSTRAINTS);
For exporting everything (tables and constraints) you can use:
// bootstrap.php
// ...
$manager->setAttribute(Doctrine_Core::ATTR_EXPORT, Doctrine_Core::EXPORT_ALL);