Adding a new attribute
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In order to add a new attribute to your magento installation, you can use following method:
Create an instance of Mage_Eav_Model_Entity_Setup class and use its method addAttribute() to add one.
$setup = new Mage_Eav_Model_Entity_Setup(’core_setup’);
$setup->addAttribute($entityTypeId, $code, $attr);
$attr is an array containing Attribute elements (like frontend and backend model, default value, etc.)
For details look at the sourcecode: