MySQL cannot update the rows in the database - php

Question

I am trying to update my selected row of data but i could not update it. Once i run the code I am getting this output {"RowCount": 0 ,"results": []}. Supposed i should get 1 but i am not getting it. Can i know how to solve this problem.
This is my PHP code:
case 'updateStudent':
$studentUpdateSQL = "UPDATE srs_student SET surname=:surname, forename=:forename,
email=:email WHERE studentid=:id";
$rs = new JSONRecordSet();
$retval = $rs->getRecordSet($studentUpdateSQL, 'ResultSet',
array(':surname'=>$surname,
':forename'=>$forename,
':email'=>$email,
':id'=>$id
));
echo $retval;
break;
This is the JSONRecordSet class:
function getRecordSet($sql, $elementName = "ResultSet", $params = null) {
$stmt = parent::getRecordSet($sql, $params);
$recordSet = $stmt->fetchAll(PDO::FETCH_ASSOC);
$nRecords = count($recordSet);
if ($nRecords == 0) {
$status = 'error';
$message = json_encode(array("text" => "No records found"));
$result = '[]';
}
else {
$status = 'ok';
$message = json_encode(array("text" => ""));
$result = json_encode($recordSet);
}
return "{\"RowCount\": $nRecords ,\"results\": $result}";
}
}

Answer 1

$rs = new JSONRecordSet();
$db_conn = pdoDB::getConnection();
$stmt=$db_conn->prepare($studentUpdateSQL);
$retval=$stmt->execute(array(':surname'=>$surname,
':forename'=>$forename,
':email'=>$email,
':id'=>$id));
echo $retval;