How to get ID of Edit User page during wp_handle_upload_prefilter, whilst in Media picker?

Question

I have an Advanced Custom Fields field group on Edit User pages (user-edit.php), containing an Image field (avatar). I aim to rename that uploaded filename to match the username of the on-page user.

My current code uses wp_handle_upload_prefilter to catch the upload, and, to ensure the rename does not happen to all WP file uploads, I think I need to check that the upload field in question is my specific ACF Image...

In the below code, (1) I can successfully get the ACF field key and field for this check (check not fully implemented)...

add_filter('wp_handle_upload_prefilter', 'custom_upload_filter' );

function custom_upload_filter( $file ) {

    // 1. GET SOURCE FIELD, COPIED FROM ACF'S MEDIA.PHP

    $field = false;
    // Search for field key within available data.
    // Case 1) Media modal query.
    if ( isset( $_POST['query']['_acfuploader'] ) ) {
        $field_key = (string) $_POST['query']['_acfuploader'];

        // Case 2) Media modal upload.
    } elseif ( isset( $_POST['_acfuploader'] ) ) {
        $field_key = (string) $_POST['_acfuploader'];
    }

    // Attempt to load field.
    // Note the `acf_get_field()` function will return false if not found.
    if ( isset( $field_key ) ) {
        $field = acf_get_field( $field_key );
    }


    // 2. GET USER'S ID AND USERNAME

    // Code here


    // 3. TEST RENAME CODE
    $file['name'] = $user_id . 'and-everything-is-awesome-' . $file['name'];
    return $file;

}

However, I'm struggling to (2) get the ID of the user on the user-edit.php page.

I have tried two methods found here on StackExchange...

1. wp_get_current_user...

This...

$user_id = (int) $user_id;
$current_user = wp_get_current_user();
if ( ! defined( 'IS_PROFILE_PAGE' ) )
    define( 'IS_PROFILE_PAGE', ( $user_id == $current_user->ID ) );

if ( ! $user_id && IS_PROFILE_PAGE )
    $user_id = $current_user->ID;
elseif ( ! $user_id && ! IS_PROFILE_PAGE )
    wp_die(__( 'Invalid user ID.' ) );
elseif ( ! get_userdata( $user_id ) )
    wp_die( __('Invalid user ID.') );

2. $_GET['user_id']

This...

// If is current user's profile (profile.php)
if ( defined('IS_PROFILE_PAGE') && IS_PROFILE_PAGE ) {
    $user_id = get_current_user_id();
// If is another user's profile page
} elseif (! empty($_GET['user_id']) && is_numeric($_GET['user_id']) ) {
    $user_id = $_GET['user_id'];
// Otherwise something is wrong.
} else {
    die( 'No user id defined.' );
}

But neither result in a user_id being located. It is always PHP Notice: Undefined variable: user_id

Is this because, whilst the Edit User page is still at user-edit.php?user_id=2, the Media modal has somehow denied us the ability to get user_id?

TL;DR - I need to get user_id of currently-editing user on user-edit.php whilst in a Media picker initiated by ACF. How can I do this?

Update with full answer

Sally CJ's answer is great. Her option #2 is optimal... use filter plupload_default_params to pass variable to the AJAX Media uploader. Then, when I do the user_avatar_rename() on filter wp_handle_upload_prefilter, the value of user_id should be ready and waiting there...

My full code below. Big thanks to Sally.

/**
 * ==============================================================================
 *             RENAME UPLOADED USER AVATAR IMAGE FILE WITH USERNAME
 *    When an image is uploaded to Edit User form through an ACF field (field_6140a6da30a17),
 *    rename file with the username of said user.
 * ==============================================================================
 */

// 1. PASS USER_ID FROM USER-EDIT.PHP TO MEDIA UPLOADER, TO GET USERNAME FOR 
// cf. https://support.advancedcustomfields.com/forums/topic/force-an-image-file-upload-to-a-particular-directory/
// cf. https://wordpress.stackexchange.com/questions/395730/how-to-get-id-of-edit-user-page-during-wp-handle-upload-prefilter-whilst-in-med/395764?noredirect=1#comment577035_395764
add_filter('plupload_default_params', function($params) {
    if (!function_exists('get_current_screen')) {
        return $params;
    }
    $current_screen = get_current_screen();
    if ($current_screen->id == 'user-edit') {
        $params['user_id'] = $_GET['user_id'];
    } elseif ($current_screen->id == 'profile') {
        $params['user_id'] = get_current_user_id();
    }
    return $params;
    });

// 2. ON UPLOAD, DO THE RENAME
// Filter, cf. https://wordpress.stackexchange.com/questions/168790/how-to-get-profile-user-id-when-uploading-image-via-media-uploader-on-profile-pa
// p1: filter, p2: function to execute, p3: priority eg 10, p4: number of arguments eg 2
add_filter('wp_handle_upload_prefilter', 'user_avatar_rename' );
function user_avatar_rename( $file ) {
    // Working with $POST contents of AJAX Media uploader
    $theuserid = $_POST['user_id'];         // Passed from user-edit.php via plupload_default_params function
    $acffield  = $_POST['_acfuploader'];    // ACF field key, inherent in $_POST
    // If user_id was present AND ACF field is for avatar Image
    if ( ($theuserid) && ($acffield=='field_6140a6da30a17') ) {
        // Get ID's username and rename file accordingly, cf. https://stackoverflow.com/a/3261107/1375163
        $user = get_userdata( $theuserid );
        $info = pathinfo($file['name']);
        $ext  = empty($info['extension']) ? '' : '.' . $info['extension'];
        $name = basename($file['name'], $ext);
        $file['name'] = $user->user_login . $ext;
        // Carry on
        return $file;
    // Else, just use original filename
    } else {
        return $file;
    }
}

Answer 1

Is this because, whilst the Edit User page is still at user-edit.php?user_id=2, the Media modal has somehow denied us the ability to get user_id?

Getting the user ID on that page is easy, e.g. you could simply use get_current_user_id() to get the ID of the currently logged-in user, or use $_GET['user_id'] to get the ID of the user for the profile that's being edited.

But the thing is, the default WordPress media uploader will upload the file via AJAX to wp-admin/async-upload.php and despite on that page, you could still get the ID of the currently logged-in user, you wouldn't be able to get the ID of the user being edited, i.e. the value of the above user_id parameter, because the media uploader doesn't "forward" the user_id value to async-upload.php.

So by default, there's no way to get that value from the wp_handle_upload_prefilter hook which runs on the async-upload.php page.

However, you could do one of these:

1. Use wp_get_referer() to get the referring URL and then read the user_id from the URL's query string.

2. Manually "forward" the user ID to async-upload.php, which can be done via the plupload_default_params filter.

Working Examples

1. For the first option above:

   // Note: This is a simplified example. In actual implementation, you'd want to
   // check if the referring page is wp-admin/user-edit.php or wp-admin/profile.php
   
   parse_str( parse_url( wp_get_referer(), PHP_URL_QUERY ), $args );
   $user_id = $args['user_id'] ?? get_current_user_id();
   

2. For the second option above:

   add_filter( 'plupload_default_params', 'my_plupload_default_params' );
   function my_plupload_default_params( $params ) {
       // the admin page's file name, without .php
       $screens = array( 'user-edit', 'profile' );
   
       if ( is_admin() && in_array( get_current_screen()->id, $screens ) ) {
           $params['user_id'] = ( defined( 'IS_PROFILE_PAGE' ) && IS_PROFILE_PAGE ) ?
               get_current_user_id() : $_REQUEST['user_id'];
       }
   
       return $params;
   }
   

   And then in your function, just use $_POST['user_id'] to get the user ID.