Make sure your script file, the file that contains the line
$currency = '€';
is encoded UTF-8. You should have an option to that effect in your editor's or IDE's "Save as" dialog.
Then make sure your connection is UTF-8 encoded as well - it is ISO-8859-1 by default.
After connecting to the database, for mySQL before 5.0.7:
mysql_query("SET NAMES utf8");
For mySQL 5.0.7 and newer:
mysql_set_charset("utf8");