Access denied when executing a SQL query from a plugin

[funyoo]

Hi,

I get this message when I enable the following plugin :

$query="SELECT username FROM user WHERE userid = 1";
$result = mysql_query($query) or die(mysql_error());

Access denied for user 'www-data'@'localhost' (using password: NO)

What could be the reason ? everything is working otherwise, the board, as the SQL queries from Maintenance > Execute SQL Query.