Use of real_escape_string with mysqli object

<?php
// Open database connection
@ $db = new mysqli('a', 'b', 'c', 'd');    CONNECTION DETAILS OMITTED
 
/*********************** 
 
 DATA CLEANING FUNCTIONS
 
 ***********************/
function clean_data($string) 
{
  if (get_magic_quotes_gpc()) $string = stripslashes($string);
  $string = htmlentities($string);	// if text contains markup, neutralize it be conversion to html entities prior to insertion
  return $db->real_escape_string($string);
}
function remove_headers($string) { 
  $headers = array("/to\:/i","/from\:/i","/bcc\:/i","/cc\:/i","/Content\-Transfer\-Encoding\:/i","/Content\-Type\:/i","/Mime\-Version\:/i"); 
  return preg_replace($headers, '', $string);
} 
 
/**********************************************
 
 CLEAN AND INSERT POST VALUES INTO THE DATABASE
 
 **********************************************/
// Clean up the form data prior to insertion in the database
$topic = $_POST['topic'];	// numeric primary key value
$nameA = clean_data($_POST['name']);
$qEmailA = clean_data($_POST['email']);	// questioner's email address goes into database
$questionA = clean_data($_POST['question']);
 
// Insert values into database
$query = "INSERT INTO expertZone_QandA VALUES ('','".$topic."','".$questionA."','','".$nameA."','".$qEmailA."')";
$result = $db->query($query); 
if ($result)
{ 
	echo '<p>ITEM ADDED</p>';
	echo '<p>$name = '.$nameA.'</p>';
	echo '<p>$qEmail = '.$qEmailA.'</p>';
	echo '<p>$question = '.$questionA.'</p>';
	echo '<p>$topic_pk = '.$topic.'</p>';
}
 
// Close database connection
$db->close();