problema - mysql_real_escape_string()

Tenho estas linha p q da o erro:

Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: Can't connect to local MySQL server through socket '/tmp/mysql.sock' (2)

if (phpversion() >= '4.3.0')

{

 $string = mysql_real_escape_string($string);

 }

 else

 {

 $string = mysql_escape_string($string);

 }