Shows how to find the optgroup of a field value in a Participants Database custom template.

pdb-single-option-value-optgroup.php

<?php
$record = new PDb_Template( $this );

// name of the field you're interested in
$field = 'interests';

// get the array of defined options
$option_list = $record->get_field_prop( $field, 'options' );

// assume the title is blank at first
$optgroup_title = '';
$value_title = '';

// scan through the options, holding on to the last option title
foreach( $option_list as $title => $value ) {
  
  if ( $value === 'optgroup' ) {
    $optgroup_title = $title;
    continue; // continue to the next option
  }
  
  // if the value matches, break and keep the last option title
  if ( $value === $record->get_value( $field ) ) {
    $value_title = $title;
    break; // we found it, break out of the loop
  }
}

// shows the result
echo $optgroup_title . ':' . $value_title;

Note that this will not work as expected with multi-select type fields because the saved value is going to be an array.

If you want to do this with a multi-select, you can get the first value from the saved values with:

current( $record->get_value( $field ) )