Hi, sadly the multibyte mode isn't exposed, so the only way to check for it is by sniffing phpinfo() which is really hacky. I wrote up a small script for you to see what I mean by hacky:
<?php
	$zend_multibyte = false;

	ob_start();
	phpinfo(INFO_GENERAL);

	$buffer = ob_get_clean();
	$pos 	= strpos($buffer, 'Zend Multibyte');
	$buffer	= substr($buffer, $pos);

	if(PHP_SAPI == 'cli')
	{
		$buffer		= ltrim($buffer);
		$zend_multibyte = (trim(substr($buffer, 25, strpos($buffer, "\n") - 25)) == 'enabled');
	}
	else
	{
		$zend_multibyte = (trim(strip_tags(substr($buffer, 28, strpos($buffer, "</tr>") - 28))) == 'enabled');
	}

	unset($buffer, $pos);

	var_dump($zend_multibyte);
?>

As for the documentation, I have added a note on the declare() construct's page that its "impossible" to determine whether the build have multibyte support or not.

Thank you!

Can you forward / re-assign the bug maybe so that it is possible to detect this during runtime, please?
Please leave a short reply, if you won't do it, so I will do this.

Done, I can add such a constant to the 5.3 branch and trunk sometime this weekend

Added, this will be available in PHP 5.3.4+ ;-)

Thanks a lot!

Someone on internals requested I added this here:

«The problem in 5.3 is not as grave as the bug report paints it, you don't need to sniff phpinfo() to see whether PHP was compiled with --enable-zend-multibyte. You can use, for instance:
 
key_exists("detect_unicode", ini_get_all())»