profile_mod.php

class Gist_Member_Extension {

  private function __construct() {
    $this->add_hooks();
  }

  function add_hooks(){
    add_filter( 'bp_signup_usermeta', [$this, 'pre_registration_filter'] );
  }

  /**
   * Mutates $usermeta to ignore "add an institution" field and inject its
   * value into "institution" field instead
   * @param  [array] $usermeta [the data from $_POST on registration page]
   * @return [array]           [possibly mutated $usermeta]
   */
  public function pre_registration_filter($usermeta){
    // debugging: output paramater
    $this->debug($usermeta, '$usermeta');

    // make sure these vars are clean for conditionals below
    $new_institution_field = FALSE;
    $institution_field = FALSE;

    // find the institution field key if we need to add a new institution
    $institution_field = array_search('Not listed', $usermeta);

    // debugging: output institution field key
    $this->debug($institution_field, '$institution_field');

    // find the new institution field ($_POST does not provide field names)
    $ids = array_reverse(explode(',',$usermeta['profile_field_ids']));
    foreach($ids as $id){
      $field = xprofile_get_field($id);
      if($field -> name == "add an institution"){
        $new_institution_field = 'field_'.$id;
        // debugging: output the field key we found
        $this->debug($usermeta[$new_institution_field], '$new_institution_field');
        // we found it so end the loop
        break;
      }
    }

    if($institution_field){
      if($new_institution_field){
        // we are adding a new instiution
        // replace institution='not listed' institution=with 'add an instution'
        $usermeta[$institution_field] = $usermeta[$new_institution_field];
        // destroy 'add an instutition' field
        unset($usermeta[$new_institution_field]);
        // debugging: output mutated field
        $this->debug($usermeta[$institution_field], '$usermeta[$institution_field]');
      }
    }

    return $usermeta;
  }
}