Magento - Add customer attribute

magento-customer-attribute.php

<?php

/* @var Mage_Customer_Model_Resource_Setup $this */
$this->addAttribute('customer', 'customer_number', array(
    'type' => 'int',
    'input' => 'text',
    'label' => 'Customer Number',
    'visible' => true,
    'required' => false,
    'is_system' => false,
    'default' => 0,
));

$attribute = Mage::getSingleton('eav/config')->getAttribute($this->getEntityTypeId('customer'), 'customer_number');
$attribute->setData('used_in_forms', array('adminhtml_customer'))
    ->save();