How to enable imageantialias on my installation?

php -r "var_dump(function_exists('imageantialias'));"
bool(false)


php5 -v 
PHP 5.3.3-7+squeeze17 with Suhosin-Patch (cli) (built: Aug 23 2013 15:06:16) 
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies
with XCache v1.3.0, Copyright (c) 2005-2009, by mOo
with Xdebug v2.1.0, Copyright (c) 2002-2010, by Derick Rethans

gd library are installed :)

Thanks!