PHP coding problem

I’m receiving an error message:

Warning: Cannot modify header information – headers already sent by (output started at /home/izacacom/public_html/blog/wp-content/themes/Izaca_blog/functions.php:11) in /home/izacacom/public_html/blog/wp-includes/pluggable.php on line 39

…when I go to my blog with the www in the url (http://www.izaca.com/blog), but it works correctly when the ‘www.’ is taken out (http://izaca.com/blog). The PHP code that its referring to is below, but I don’t know how to fix it.

if ( !function_exists('wp_redirect') ) :
function wp_redirect($location, $status = 302) {
	global $is_IIS;

	$location = apply_filters('wp_redirect', $location, $status);

	if ( !$location ) // allows the wp_redirect filter to cancel a redirect
		return false;

	$location = wp_sanitize_redirect($location);

	if ( $is_IIS ) {
		header("Refresh: 0;url=$location");
	} else {
		if ( php_sapi_name() != 'cgi-fcgi' )
			status_header($status); // This causes problems on IIS and some FastCGI setups
		header("Location: $location");