Users_Table_Last_Seen.php

<?php

/*
Plugin Name: Users Table Last Seen
Plugin URI: http://samelh.com
Description: updates users last seen status each time they browse the front-end and adds last seen to users table with user last seen time difference 
Author: Samuel Elh
Version: 0.1
Author URI: http://samelh.com
*/

// hook into wp to run internally in the front-end
add_action('wp', 'se_update_user_last_seen');

/**
  * Update user last seen with current time int
  *
  * @param $user_id int user ID to update (optional)
  * @return null
  */

function se_update_user_last_seen( $user_id = 0 ) {
	if( ! is_user_logged_in() )
		return; // user is logged out

	if( ! $user_id ) // get current user ID
		$user_id = wp_get_current_user()->ID;

	// update user meta
	update_user_meta( $user_id, 'se_user_last_seen', time() );
}

// register the new column
add_filter( 'manage_users_columns', 'se_filter_users_columns' );

// append our new column to the existing ones
function se_filter_users_columns( $columns ) {
    $columns['last_seen'] = 'Last seen';
    return $columns;
}

// filter columns content to set ours 
add_filter( 'manage_users_custom_column', 'se_append_users_column_data', 10, 3 );

/**
  * Set last_seen column content with the user last seen data
  *
  * @param $column_value str the content of the selected column
  * @param $column_name str the name of the column
  * @param $user_id int the selected user ID
  * @return str column value
  */

function se_append_users_column_data( $column_value, $column_name, $user_id ) {
	// getting the meta from the database
    $meta = (int) get_user_meta( $user_id, 'se_user_last_seen', TRUE );
    $diff = $meta > 0 ? se_user_last_seen_diff( $meta ) . ' ago' : 'n/a';

    if( 'last_seen' == $column_name )
    	return $diff;

    return $column_value;
}

/**
  * Get the date difference based on a specific time integer
  *
  * @param $targetDate int the date to target (time integer)
  * @param $nowStr str when the difference is less than a second (very recent)
  * @return str difference
  */

function se_user_last_seen_diff( $targetDate, $nowStr = 'a moment' ) {

	$targetDate = new DateTime( date("Y-m-d H:i:s", $targetDate) );
	$now = new DateTime( date("Y-m-d H:i:s", time()) );
	$delta = $now->diff($targetDate);
	$str = '';
	$quantities = array(
	    'year' => $delta->y,
	    'month' => $delta->m,
	    'day' => $delta->d,
	    'hour' => $delta->h,
	    'minute' => $delta->i,
	    'second' => $delta->s
	);

	foreach($quantities as $unit => $value) {
	    if($value == 0) continue;
	    if($value != 1) {
	        $unit .= 's';
	    }
	    $str .= $value . ' ' . $unit;
	    $str .=  ', ';
	    break;
	}

	return $str == '' ? $nowStr : substr($str, 0, -2);
	
}