Is it necessary to use mysql_real_escape_string(), when magic_quotes_gpc is on?

Question

To prevent SQL injection, is it necessary to use mysql_real_escape_string(), when magic_quotes_gpc is on?

Answer 1

For some rare encodings, such as GBk - yes.
But you should revert it not for this reason. Magic quotes should be turned off anyway (and will be in the next PHP version). So, mysql_real_escape_string() is the only escape function is left. Note that it is not sql injection prevention function. Many many people don't understand this point: it's just a part of syntax. It must be used not to "protect" anything, but to assemble syntactically correct SQL query. And must be used every time you build your query, no matter where data come from. Sure it will protect you from SQL injections too, as a side effect.
Of course, mysql_real_escape_string() works only within quoted strings. So, if you do

$num=mysql_real_escape_string($num);
$sql="SELECT INTO table SET data=$num"; /BAD!!!

It will protect nothing. If you going to use numbers unquoted, it must be cast to the proper type obligatory, like this:

$num=intval($num);
$sql="SELECT INTO table SET data=$num"; /GOOD

• Keep in mind that mo make mysql_real_escape_string() works as intended, proper client encoding should be set, and it is possible only with mysql_set_charset() function, SET NAMES query will not set that.

If you want to get rid of all these complexities, you can use prepared statements, though you will need to switch your mysql driver to mysqli or PDO.

Please note that no proper syntax nor prepared statements would not help you with query parts other than literals. You can't escape Identifiers or operators. If you happen to use these parts dynamically, they must be hardcoded in your script, like this (for the ORDER BY clause):

$orders=array("name","price","qty");
$key=array_search($_GET['sort'],$orders));
$orderby=$orders[$key];
$query="SELECT * FROM `table` ORDER BY $orderby";

or this (WHERE clause)

$w=array();
if (!empty($_GET['rooms'])) $w[]="rooms='".mysql_real_escape_string($_GET['rooms'])."'";
if (!empty($_GET['space'])) $w[]="space='".mysql_real_escape_string($_GET['space'])."'";
if (!empty($_GET['max_price'])) $w[]="price < '".mysql_real_escape_string($_GET['max_price'])."'";

if (count($w)) $where="WHERE ".implode(' AND ',$w); else $where='';
$query="select * from table $where";