This piece of code in the WordPress codex and put it into functions.php:

if ( get_magic_quotes_gpc() ) {
$_GET = stripslashes_deep($_GET );
$_POST = stripslashes_deep($_POST );
$_COOKIE = stripslashes_deep($_COOKIE);

The existence of magic quotes has been a headache for many PHP composers. Future versions of PHP may very likely deprecate them. Code will, however, need to continue to work around them as long as PHP4 and PHP5 remain in use.

Disabling magic quotes in .htaccess file:
php_flag magic_quotes_gpc Off

More detail