我们如何编写PHP脚本来释放与MySQL结果关联的游标内存？

<?php
   $dbhost = &#39;localhost:3036&#39;;
   $dbuser = &#39;root&#39;;
   $dbpass = &#39;rootpassword&#39;;
   $conn = mysql_connect($dbhost, $dbuser, $dbpass);
   
   if(! $conn )  {
      die(&#39;Could not connect: &#39; . mysql_error());
   }
   
   $sql = &#39;SELECT tutorial_id, tutorial_title, tutorial_author, submission_date
      FROM tutorials_tbl&#39;;
   mysql_select_db(&#39;TUTORIALS&#39;);
   $retval = mysql_query( $sql, $conn );

   if(! $retval ) {
      die(&#39;Could not get data: &#39; . mysql_error());
   }

   while($row = mysql_fetch_array($retval, MYSQL_NUM)) {
      echo "Tutorial ID :{$row[0]} <br> ".
         "Title: {$row[1]} <br> ".
         "Author: {$row[2]} <br> ".
         "Submission Date : {$row[3]} <br> ".
         "--------------------------------<br>";
   }
   mysql_free_result($retval);
   echo "Fetched data successfully</p><p>";
   mysql_close($conn);
?>