wordpress - get array of attachment IDs

////  We use $gallery_media as an array containing all attachment IDs that have been approved by the admin before
    $args = array(
        'post_type' => 'attachment',
        'posts_per_page' => -1,
        'orderby'          => 'post_date',
        'order'            => 'ASC',
        'post_status' =>'any',
        'post_parent' => $post->ID,
        // This queries a custom field I created for media attachments – can be either 'piúblished' or 'pending'
        'meta_query' => array(
            array(
                'key' => 'attachment_status',
                'value' => 'published',
            )
        )
        );
    $attachments = get_posts( $args );
    if ( $attachments ) {
        $gallery_media = array();
        foreach ( $attachments as $attachment ) {
            // Check if attachment has been reviewed and custom status is "published"
            // Put together our array
            $gallery_media[] = apply_filters( 'id' , $attachment->ID );
        }
    }

// Outputs an indexed array containing attachment IDs according to our query