with the intl extension in PHP 5.3+, you can use the NumberFormatter class:
$amount = '12345.67';
$formatter = new NumberFormatter('en_GB', NumberFormatter::CURRENCY);
echo 'UK: ', $formatter->formatCurrency($amount, 'EUR'), PHP_EOL;
$formatter = new NumberFormatter('de_DE', NumberFormatter::CURRENCY);
echo 'DE: ', $formatter->formatCurrency($amount, 'EUR'), PHP_EOL;
which prints :
UK: €12,345.67
DE: 12.345,67 €