$relationshipTypes = CRM_Core_PseudoConstant::get('CRM_Contact_BAO_Relationship', 'relationship_type_id');
$relationshipType = CRM_Core_PseudoConstant::getKey('CRM_Contact_BAO_Relationship', 'relationship_type_id', 'Child of');
$relationships = civicrm_api3('Relationship', 'get', [ 'sequential' => 1, 'relationship_type_id' => CRM_Core_PseudoConstant::getKey('CRM_Contact_BAO_Relationship', 'relationship_type_id', 'Child of'), 'contact_id' => 1 ]);In this code example, the `civicrm_api3()` function is used to retrieve all the relationships of the 'Child of' relationship type for the contact with ID 1. The `getKey()` function is used to retrieve the relationship type ID for 'Child of'. In conclusion, the CRM_Core_PseudoConstant relationshipType is a package/library in the CiviCRM PHP library that provides predefined relationship types for CiviCRM entities. We can use its functions to retrieve relationship types or to filter relationships based on relationship types.