Problem in profile.php

[burnist]

In profile.php there exsists this piece of code

PHP Code:

$vbulletin->input->clean_array_gpc('p', array(
        // coppa stuff
        'coppauser'    => TYPE_BOOL,
        'parentemail'  => TYPE_STR,
        // IM handles / homepage
        'aim'          => TYPE_STR,
        'yahoo'        => TYPE_STR,
        'icq'          => TYPE_STR,
        'msn'          => TYPE_STR,

        'homepage'     => TYPE_STR,
        // user title
        'resettitle'   => TYPE_STR,
        'customtext'   => TYPE_STR,
        // birthday fields
        'day'          => TYPE_INT,
        'month'        => TYPE_INT,
        'year'         => TYPE_INT,
        'oldbirthday'  => TYPE_STR,
        // redirect button
        'gotopassword' => TYPE_NOCLEAN,
        // custom profile fields
        'userfield'    => TYPE_ARRAY,
    )); 


Now, I am currently trying to add a field to the user table that is edited through profile (trying to add a Skype listing alongside msn, aim etc).
I have added the field to $validfields using a hook (userdata_start) and set the userdata and GPC in another (profile_updateprofile). The addition of the field only works if it is also present in the code block above (have tryed it with the code hacked in and it works) and have no idea how to add it there using a hook.

If you can help I will release the mod here and this is bound to be useful refrence for others

Also what does

$vbulletin->input->clean_array_gpc('p', array()); do? and what part does the 'p' play?