Our code should never use magic numbers (for example to represent values of option values or option groups) or specific custom field and custom table names. Instead, the CiviCRM coding standard is to create a constant to represent the value, and then use the constant in the code. Always use ALLCAPS for constant names.
For example:
define( "COURSE_CONFERENCE_TYPE", 'course_conference_type_21' );
....
 if (CRM_Utils_Array::value(COURSE_CONFERENCE_TYPE, $query->_returnProperties)) {