I have very simple function to convert some sumbols to underscores:

function to_tag($text)
{	
	$from=" \"\t\\/:#$%^&*()=+-!`'~?<>[],.";
	$retval=strtr($text,$from,str_repeat('_',strlen($from)));
              // debug output
	echo "\r\n<BR>[$text] >> [$retval]";
	return $retval;
}

And I have such string (first world is russian, Win-1251): "?????????  d'fusion"

On simple example all is Ok:
<?
	include_once('tag.inc.php');
	to_tag("?????????  d'fusion");
?>

X-Powered-By: PHP/4.0.7-dev
Content-type: text/html; charset=windows-1251


<BR>[?????????  d'fusion] >> [?????????__d_fusion]

But!
If this function called from more complex real code, ['] doesn't converted:

 Function using to_tag():

	function getPropXML($FID,$TID)
	{
		debug("getPropXML($FID,$TID)");
		$value=$this->getProp($FID,$TID);
		$target=$this->getCopy($TID);
		return t(to_tag($target->Name)
			,t('path',$this->getPath($TID))
			.t('value',cdata($value))
			.t('Parent',$this->getPath($target->ParentID)));
	}

Debug output for object with this string:
...
<BR>[????????] >> [????????]
<BR>[????????? d'fusion] >> [?????????_d'fusion]
<BR>[$25-70] >> [_25_70]
...

I tried to clear situation and find point of bug, but can't :(