mysql_real_escape_string

I got this weird message in a PHP code where I wanted to escape input before putting it in a query:

Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: Access denied for user 'XXX'@'localhost' (using password: NO) in XXX on line 7


Warning: mysql_real_escape_string() [function.mysql-real-escape-string]: A link to the server could not be established in XXX on line 7

It seems that this function needs a database connection to be opened in order to work. The reason is, that it must know the character set of the database in order to protect from SQL Injection.