removing blank lines from XHTML

I have XHTML that has empty (blank) lines:

some_tag

empty line

empty line

empty line

some_tag

I wonder how to get rid of them using PHP str_replace. The result should be开发者_如何学C some_tag some_tag. What are these blank lines anyways?

$output = str_replace(array("\n", "\r"), '', $input);

OK, after that everything will be in one line. Hope that's ok. Otherwise you'd have to use regular expressions to get rid only of line breaks which are not within tags.