eatlas_field_extra_attributes.module

<?php
define('EATLAS_FIELD_EXTRA_ATTRIBUTES_FIELD_NAME', 'field_extra_attributes_location');

/**
 * Implement: hook_form_FORM_ID_alter
 * For form ID: field_ui_field_edit_form (the form used to edit fields)
 *     => hook_form_field_ui_field_edit_form_alter
 * See:
 *     https://api.drupal.org/api/drupal/modules!field_ui!field_ui.api.php/function/hook_field_settings_form/7
 */
function eatlas_field_extra_attributes_form_field_ui_field_edit_form_alter(&$form, &$form_state, $form_id) {
	// Define the form field
	// NOTE: '#weight' can be used to control where the field appear in the form.
	$locationField = array (
		'#type' => 'textfield',
		'#title' => 'Location',
		'#default_value' => '',
		'#description' => 'Define where this field has to be displayed.<br/>' .
				'Valid values: sidebar_first<br/>' .
				'<em>NOTE: This field was added by the "eatlas_field_extra_attributes" module.</em>'
	);

	// Edition mode: Retrieve the saved value (for some reason, this is not done automatically)
	$mode = !isset($form_state['input']) || empty($form_state['input']) ? 'EDIT' : 'SAVE';
	if ($mode === 'EDIT' &&
			isset($form['#field']['settings'][EATLAS_FIELD_EXTRA_ATTRIBUTES_FIELD_NAME]) &&
			$form['#field']['settings'][EATLAS_FIELD_EXTRA_ATTRIBUTES_FIELD_NAME]) {
		$locationField['#default_value'] = $form['#field']['settings'][EATLAS_FIELD_EXTRA_ATTRIBUTES_FIELD_NAME];
	}

	// Add the form field to the form
	// NOTE: Drupal manage the save
	$form['field']['settings'][EATLAS_FIELD_EXTRA_ATTRIBUTES_FIELD_NAME] = $locationField;
}

/**
 * API Function
 * Return an array containing all field names (String)
 * that should be displayed in '$location' region.
 */
function eatlas_field_extra_attributes_get_content_field_names($content, $location) {
	$contentFieldNames = array();
	if (!$content || !$location) {
		return $contentFieldNames;
	}

	$location = _eatlas_field_extra_attributes_clean_str($location);

	// Sort the content elements (it's done by "element_children"
	//     method when TRUE is sent as 2nd parameter)
	$sortedFieldNames = element_children($content, TRUE);

	foreach($sortedFieldNames as $field_name) {
		$field_info = field_info_field($field_name);
		if (isset($field_info['settings'][EATLAS_FIELD_EXTRA_ATTRIBUTES_FIELD_NAME])) {
			$extraAttribute = _eatlas_field_extra_attributes_clean_str($field_info['settings'][EATLAS_FIELD_EXTRA_ATTRIBUTES_FIELD_NAME]);
			if (strcasecmp($extraAttribute, $location) === 0) {
				$contentFieldNames[] = $field_name;
			}
		}
	}

	return $contentFieldNames;
}

function _eatlas_field_extra_attributes_clean_str($str) {
	// Remove none alphanumeric characters to be more robust
	return preg_replace("/[^A-Za-z0-9]/", '', $str);
}

?>