html_entity_decode utf-8 and iso-8859-1

[blackhorse]

echo 'iso-8859-1 encode <br />';
	$var=htmlentities('< > & \' "', ENT_QUOTES, 'iso-8859-1');
	echo 'iso decode: <br />';
	echo html_entity_decode($var, ENT_QUOTES, 'iso-8859-1');
	echo '<br />';
	echo 'utf decode: <br />';
	echo html_entity_decode($var, ENT_QUOTES, 'utf-8');

So if I use iso-8859-1 htmlentities('< > & \' "', ENT_QUOTES, 'iso-8859-1'), I can still use utf8 html_entity_decode($var, ENT_QUOTES, 'utf-8') to covert them back.

But if I use utf-8 htmlentities, then when I use iso-8859-1 htmle_entity_decode, some of the unicode htmlentities will not be coverted back.

I am working with 3rd party database, I need to html_entity_decode the values. Due to I don't know if the original htmlentities character set is iso-8859-1 or utf-8, I am using html_entity_decode($var, ENT_QUOTES, 'utf-8'), am I missing something here?

Thanks!