Allow to set subject and body with query arguments

I am not sure if I exactly understand what you mean.

There are several ways to act or hook in when a message is sent (hook_form_alter, our own hooks, see http://drupal.org/node/369399). If you know enough about PHP, this shouldn't be a real problem. You could either display default values with hook_form_alter or append/prepend/replace something directly in the message before it's getting saved. The only thing you need to figure out is how to get your data you want to insert.

What we could provide is a easy way to set the subject/body to default values based on GET Parameters.

Feel free to ask specific questions about the implementation if you have any.

The easiest way to do this is something like the following:

1. Add a link to privatemsg which somehow contains the nid you want to refer to. For example, you can set use messages/new/your_uid?destination=/node/123. If you do this, the user will also be redirected back to your node when the private message is sent.

2. Either create a new module mymodule or put the following code inside one of your own modules (rename mymodule to the name of your module)

function mymodule_form_privatemsg_list_alter(&$form, &$form_state) {  
  // check if there is a destination argument
  if (isset($_REQUEST['destination'])) {
    // check if the first part is node and the second a node id that can be loaded with node_load()
    if ((arg(0, $_REQUEST['destination']) == 'node') && ($node = node_load(arg(1, $_REQUEST['destination']))) 
        // set default value to [node-type: node-title]
       $form['privatemsg']['subject']['#default_value'] = t('[@type: @name]', array('@type' => $node->type, '@name' => $node->title));
    }
  }
}

The code is provided without warranty and is untested :)

We may include a easier way to set the subject in the future, but you still need to load the node so that you know the type and name.

I am changing the issue a bit to make it a feature request for adding a subject variable..

So currently the only thing that can be passed in the URL is "destination"?

Tested the solution above and it broke the site :(

Will check if syntax and everything was correct...

Thanks this can be very useful.

<?php
/**
 * Pass RE:[node title] as privatemsg subject field.
 */

function mymodule_form_privatemsg_new_alter(&$form, &$form_state) { 
  // check if there is a destination argument
  if (isset($_REQUEST['destination'])) {
    // check if the first part is node and the second a node id that can be loaded with node_load()
    if ((arg(0, $_REQUEST['destination']) == 'node') && ($node = node_load(arg(1, $_REQUEST['destination'])))) {
        // set default value to [node-type: node-title]
       $form['privatemsg']['subject']['#default_value'] = t('RE:[@type - !name]', array('@type' => $node->type, '!name' => $node->title));
    }
  }
}
?>

What code are you using for the link?

It has to look something like this:

"http://yoursite.com/messages/new/" . $node->uid . "?destination=node/" . $node->nid

#398916: Remove hook_form_alter in favor of a more flexible privatemsg_new form will allow to set the subject with a GET argument, setting this to duplicate.