Mysqli insert_id()

You should use it as follow:

<?php
$mysqli = new mysqli(SQLI_SERVER, MYSQLI_USER, MYSQLI_PWD, MYSQLI_DBNAME);
if ($result = $mysqli->query("INSERT INTO user(f_name, l_name) VALUES('$f_name', '$l_name');")) {
   echo 'The ID is: '.$mysqli->insert_id;
}
?>

The mysqli_insert_id() function returns the ID generated by a query on a table with a column having the AUTO_INCREMENT attribute. If the last query wasn't an INSERT or UPDATE statement or if the modified table does not have a column with the AUTO_INCREMENT attribute, this function will return zero.

Looks like you don't have a field with AUTO_INCREMENT attribute or if you have one you are not using it in the query.