V appears to be an undocumented option for input:
   'input'   => 'V',

To Do: experiment with various input settings:

   'input'   => 'R',
   'input'   => 'V',
   'input'   => 'VR',

One user found that adding a bogus 'sql' option to his virtual field resulted
in success, where other attempts at using the virtual field were unsuccessful.

-----

Miscellaneous examples follow.

$opts['fdd']['dummy1'] = array (
   'input'   => 'R',
   'name'    => 'Service Link',
   'sql|VFL' => 'CONCAT("<a href=\"my_facility_events_2011.php?PME_sys_fl=1&PME_sys_fm=0&PME_sys_sfn[0]=1&PME_sys_sfn[1]=-2&PME_sys_sfn[2]=3&PME_sys_operation=PME_op_View&PME_sys_rec=", service_id, "\" target=\"_blank\">Service</a>")',
   'options' => 'VFL',
   'escape'  => false,
   'sort'    => false
);

$opts['fdd']['dummy2'] = array (
   'name' => 'Thumbnail',
   'sql|VLF' => 'if(category = "other",
      CONCAT("<a href=\"", dir, "/", filename, "\" target=\"_blank\">link</a>"),
         if(category = "thumb", CONCAT("<a rel=\"example_group\" href=\"", REPLACE(dir, "/tn", "/"), filename, "\" target=\"_blank\"><img src=\"", dir, "/", filename, "\" ", " alt=\"\" border=\"0\"></a>"), ""))',
   'options'  => 'VLF',
   'input'  => 'R',
   'escape' => false,
   'sort'     => false
);

$opts['fdd']['dummy3'] = array (
   'name' => 'Replace',
   'sql|VLF' => 'if(category = "thumb", CONCAT("<a href=\"fancybox.change.php?upld_id=", upld_id, "\" target=\"_blank\">Replace</a>"), "")',
   'options'  => 'VLF',
   'input'  => 'R',
   'escape' => false,
   'sort'     => false
);

// Virtual (dummy) fields for List and View modes facilitate displaying calculations
// or other bits of information that do not physically exist in the data table.
// examples: http://platon.sk/forum/projects/viewtopic.php?t=1046&highlight=virtual

// -----

// `domain` is another field in this table

$opts['fdd']['dummy1'] = array (
   'input'   => 'R',
   'name'    => 'Whois',
   'sql'     => 'CONCAT("<a href=\"http://whois.domaintools.com/", domain, "\" target=\"_blank\">whois</a>")',
   'options' => 'FL',
   'escape'  => false,
   'sort'    => true
);

// -----

$opts['fdd']['dummy2'] = array (
   'input'   => 'R',
   'name'    => 'Google',
   'sql'     => 'CONCAT("<a href=\"http://www.google.com/\" target=\"_blank\">Google</a>")',
   'options' => 'FL',
   'escape'  => false,
   'sort'    => true
);

// -----

// Look up last_name and first_name where both tables have a user/user_name field
// Regular field with lookup

$opts['fdd']['user'] = array(
  'default'    => '',
  'input'      => '',
  'maxlen'     => 20,
  'name'       => 'User Name',
  'options'    => 'ACPVDFL',
  'required'   => true,
  'select'     => 'D',
  'size|ACP'   => 20,
  'sqlw'       => 'TRIM("$val_as")',
  'sort'       => true,
  'values'     => array(
    'table'       => 'vs_users',
    'column'      => 'user_name',
    'description' => array(
       'columns'  => array('0' => 'last_name', '1' => 'first_name'),
       'divs'     => array('0' => ', '),
     ),
    'filters'     => 'hidden = "0" AND deleted = "0"',
    'orderby'     => 'last_name')
);

// Dummy/Virtual field looks up user level, displaying it in lighter color
// PMEjoin1 was created above when looking up last_name and first_name

$opts['fdd']['virtual_1'] = array(
   'colattrs' => 'style="color:#999999;background-color:transparent"',
   'input'    => 'VR',
   'name'     => 'UL',
   'options'  => 'LFV',
   'sql'      => 'PMEjoin1.user_level'
);


// -----

// Create below the dummy /virtual field "dummy1"
// which in View mode displays code for a database connection

$opts['fdd']['mysql_db_name'] = array(
  'default'    => '',
  'input'      => '',
  'maxlen'     => 255,
  'name'       => 'MySQL DB',
  'options'    => 'ACPVD',
  'required'   => false,
  'select'     => 'T',
  'size|ACP'   => 110,
  'sqlw'       => 'TRIM("$val_as")',
  'sort'       => true
);

$opts['fdd']['mysql_username'] = array(
  'default'    => '',
  'input'      => '',
  'maxlen'     => 255,
  'name'       => 'MySQL User',
  'options'    => 'ACPVD',
  'required'   => false,
  'select'     => 'T',
  'size|ACP'   => 110,
  'sqlw'       => 'TRIM("$val_as")',
  'sort'       => true
);

$opts['fdd']['mysql_pwd'] = array(
  'default'    => '',
  'input'      => '',
  'maxlen'     => 255,
  'name'       => 'MySQL Pwd',
  'options'    => 'ACPVD',
  'required'   => false,
  'select'     => 'T',
  'size|ACP'   => 110,
  'sqlw'       => 'TRIM("$val_as")',
  'sort'       => true
);

$opts['fdd']['dummy1'] = array (
   'escape'  => false,
   'name'    => 'db conn',
   'options' => 'V',
   'sort'    => false,
   'sql'     => 'if(mysql_db_name <> "",
      CONCAT(
         "\$opts[\'db\'] = \'", mysql_db_name, "\';
         \$opts[\'un\'] = \'", mysql_username, "\';
         \$opts[\'pw\'] = \'", mysql_pwd, "\';
         \$persistent = \'\';
         if(\$persistent == (bool) @ini_get(\'allow_persistent\')){
            if(!\$opts[\'dbh\'] = @mysql_pconnect(\$opts[\'hn\'], \$opts[\'un\'], \$opts[\'pw\'])){
               die(mysql_error());
            }
         }else{
            if(!\$opts[\'dbh\'] = @mysql_connect(\$opts[\'hn\'], \$opts[\'un\'], \$opts[\'pw\'])){
               die(mysql_error());
            }
         }
         if(!@mysql_select_db(\$opts[\'db\'], \$opts[\'dbh\'])) {
            die(mysql_error());
         }if(isset(\$opts[\'tb\']) && !\$table_exists = @mysql_query(\'SELECT 1 FROM `\'.\$opts[\'tb\'].\'` LIMIT 0\')){
            die(mysql_error());
         }if(!@mysql_query(\'SET NAMES UTF8\')){ die(mysql_error()); }
         if(!@mysql_query(\'SET COLLATION_CONNECTION=UTF8_GENERAL_CI\')){ die(mysql_error()); }"
      ),
       "")'
);




// -----


$opts['fdd']['dummy1'] = array (
   'name' => 'Stats',
   'sql' => 'if(host_name = "aqhost.com",
      CONCAT("<a href=\"http://",
         domain_name, ":2082/3rdparty/phpMyAdmin/index.php\" target=\"_blank\">pma</a><br><a href=\"http://", domain_name,
            ":2082/frontend/AQHost/backup/index.html\" target=\"_blank\">backup</a><br><a href=\"http://",
               domain_name,
                  ":2082/frontend/AQHost/stats/index.html\" target=\"_blank\">stats</a><br><a href=\"http://",
                     domain_name,
                        ":2082/frontend/AQHost/cron/index.html\" target=\"_blank\">cron</a>"),
                           "")',
   'options'  => 'LF',
   'escape' => false,
   'sort'     => false
);


$opts['fdd']['dummy']['sql'] = 'User1 + User2';

$now = time();

$opts['fdd']['dummy_field'] = array(
  'css'        => array('postfix' => 'right-justify'),
  'name'       => 'Days since last edit',
  'mask'       =>  '%0.1f',
  'options'    => 'L',
  'sql'        => "($now - UNIX_TIMESTAMP(updated))/86400",
);


$opts['fdd']['dummy1'] = array (
   'name'    => 'Dummy',
   'sql'     => 'CONCAT(first_name, " ", last_name)',
   'options' => 'RDVL',
   'escape'  => false,
   'sort'    => true
);


$opts['fdd']['CurrentDate'] = array(
  'name'     => 'Current Date',
  'select'   => 'T',
  'maxlen'   => 14,
  'options'  => 'L',
  'sql'    => 'CURDATE()',
  'sort'     => true
);

/*
3. Virtual field

PME principle is : Each hash-entry of the $opts['fdd'] table in
the calling script is a field from the main table. But, in some
case, you may want to display something LIKE a field which isn't
a field in the main table (computed field, joined field, ...)
That's why we add the 'V' flag to 'input' option.

4. custom join: Even for basic "table lookup", you may require
to do some custom join. this is the purpose of this. Custom
join is required when you use inside a virtual field because
PME can't guess how to join. (the joining "field" doesn't exist)

5. sql feature. (See the documentation) It's especially
useful for virtual fields (else PME won't know what to display)

6. your sample :

See the sql script uploaded to see the model I used.
 */

$debug_query = true; //useful trick to know

$opts['hn'] = 'localhost';
$opts['un'] = 'root';
$opts['pw'] = '';
$opts['db'] = 'trip';
$opts['tb'] = 'trips'; // main table

$opts['key'] = 'tdyrefnum'; // Name of field which is the unique key
$opts['options'] = 'ACPVDF';
$opts['navigation'] = 'DUGB';

$opts['fdd']['tdyrefnum'] = array( //'primary key' of 'trips'
'name' => 'Tdyrefnum',
'required' => true,
'sort' => true
);
$opts['fdd']['label'] = array( //sample field of 'trips'
'name' => 'Label',
'maxlen' => 45,
'required' => true,
'sort' => true
);

//travellers' table part

//do the join
$opts['fdd']['join_to_travellers'] = array( //join_to_travellers or foo or dummy or...
   'input' => 'VR', // ........ because field is V(irtual), it don't exist in the main table
   'options' => '', //just do the join, don't display anything
   'values' => Array( //API for currently making a join in PME.
   'table' => 'travellers', //the joined table
   'column' => 'idtraveller', //the joined key
   'description' => 'idtraveller', //the joined value - here, it's only required to force the join
   'join' => '$main_table.idtraveller = $join_table.idtraveller') //custom join because PME can't guess the joining field because it's virtual.
);

//sample of display of a field in the joined table

$opts['fdd']['a_field_from_traveller'] = array(
   'name' => 'FirstName',
   'input' => 'VR',
   'options' => 'LFV',
   'sql' => 'PMEjoin2.label'
   //joined table are aliased and numbered starting from 0 and using the field rank
   // join to travellers is the field number 2 so the join performed is PMEjoin2
);

$opts['fdd']['join_to_requesters'] = array(
   'input' => 'VR',
   'options' => '',
   'values' => Array(
     'table' => 'tripsrequesters',
     'column' => 'tdyrefnum',
     'description' => 'tdyrefnum',
     'join' => '$main_table.tdyrefnum = $join_table.tdyrefnum'
   )
);

$opts['fdd']['a_field_from_tripsrequesters'] = array(
   'name' => 'label from tripsrequester',
   'input' => 'VR',
   'options' => 'LFV',
   'sql' => 'PMEjoin4.label'
);

$opts['fdd']['join_to_tripstravellers'] = array(
   'input' => 'VR',
   'options' => '',
   'values' => Array(
   'table' => 'tripstravellers',
   'column' => 'tdyrefnum',
   'description' => 'tdyrefnum',
   'join' => '$main_table.tdyrefnum = $join_table.tdyrefnum')
);

$opts['fdd']['a_field_from_tripstravellers'] = array(
   'name' => 'label from tripstraveller',
   'input' => 'VR',
   'options' => 'LFV',
   'sql' => 'PMEjoin6.label'
);

// Now important call to phpMyEdit
require_once 'phpMyEdit.class.php';
new phpMyEdit($opts);


/*
2004-05-13
In the CVS version of PME you can set a field as being a virtual
field so it doesn't need to be in the database. Have a look
at the documentation in the CVS for the full options.

Dummy fields can be configured to make calculations, for example
to add 30 to 'col_name':
 */

$opts['fdd']['dummy'] = array(
  'name'     => 'Dummy',
  'options'  => 'L',
  'select'   => 'T',
  'sql'    => 'some_other_column + 30',
  'sort'     => true
);

// ------

$opts['fdd']['dummy'] = array(
  'options'  => 'L',
  'name'     => 'Total',
  'select'   => 'T',
  'maxlen'   => 11,
  'sql'      => 'field1 + field2',
  'sort'     => true
);

// -----

Dummy fields can be created to display calculations.

$opts['fdd']['due_date'] = array(
  'input'      => '',
  'maxlen'     => 10,
  'name'       => 'Due Date',
  'options'    => 'L',
  'select'     => 'T',
  'sort'       => true
);

$opts['fdd']['dummy_column'] = array(
  'maxlen'   => 14,
  'name'     => 'Days Until',
  'options'  => 'L',
  'select'   => 'T',
  'sort'     => true,
  'sql'    => 'due_date - CURDATE()'
);

// -----

$now = time();

// updated field is TIMESTAMP column type

$opts['fdd']['dummy_field'] = array(
  'css'        => array('postfix' => 'right-justify'),
  'name'       => 'Days since last edit',
  'mask'       =>  '%0.1f',
  'options'    => 'L',
  'sql'        => "( $now - UNIX_TIMESTAMP(updated) ) / 86400",
);


// -----

// http://platon.sk/forum/projects/viewtopic.php?t=961&highlight=lookup

$opts['fdd']['client'] = array(
  'name'     => 'Account Name',
  'values'   => Array(
     'table'       => 'client', // joined table
     'column'      => 'id', // the joined key
     'description' => 'ID'),
  'join'     => '$main_table.billing = $join_table.client',
  'options'  => 'LVD',
  'maxlen'   => 1,
  'sort'     => false,
  'sql'      => 'PMEjoin2.client'
);

$opts['fdd']['virtual_1'] = array(
  'colattrs' => 'align="center"',
  'default'  => '0',
  'maxlen'   => 11,
  'name'     => 'Account Number',
  'options'  => 'LACPVDR',
  'select'   => 'T',
  'sort'     => true,
  'sql'      => 'PMEtable0.client'
);