As you asked for an example, check this code:-
<?php
session_start(); // start session to get SESSION variable value
error_reporting(E_ALL); // catch all type of errors
ini_set('display_errors',1); // display those errors
$conn = mysqli_connect('servername','username','password','databasename'); // fill your database details here
$user_id = mysqli_real_escape_string($conn,$_SESSION['userid']);// prevent form SQL injection
$last_visit = date('Y-m-d H:i:s'); // you can change date format according to your wish
$sql = "UPDATE users SET last_visit = '". $last_visit ."' WHERE id = '". $user_id ."'"; // change in query
$result = mysqli_query($conn,$sql) or die(mysqli_error($conn)); // execute query and catch error if any happen
if($result){
echo "Record updated successfully";
}else{
echo 'Some error occur while updating. Please try again.';
}
?>
Note:- just put your database credentials and go-on.