PHP 5.3 can't find normalizer_normalize()

Question

I am trying to use the normalizer_normalize() function introduced in PHP 5.3 (says the doc), however I can't use it:

$ php -r 'echo normalizer_normalize("tést");'
PHP Fatal error:  Call to undefined function normalizer_normalize()
in Command line code on line 1

I've checked my PHP version but it's 5.3:

$ php --version
PHP 5.3.6 (cli) (built: Sep 12 2011 18:02:42)

I don't understand why PHP can't find it?