XMLReader - http://www.php.net/manual/en/book.xmlreader.php .
As far as I know
XMLReader streams the file rather than reading the whole file in before doing anything with it.
From there you can convert it to a
simplexml object if you would prefer it like http://posterous.richardcunningham.co.uk/using-a-hybrid-of-xmlreader-and-simplexml
Not sure about a Symfony specific component though.