How can I set text value of SimpleXmlElement without using its parent?

You can do with a SimpleXMLElement self-reference:

$firstC->{0} = "Victory!!"; // hackity, hack, hack!
//  -or-
$firstC[0]   = "Victory!!";

found after looking at

var_dump((array) reset($xml->xpath("(//c)[3]")))

This also works with unset operations as outlined in an answer to:

• Remove a child with a specific attribute, in SimpleXML for PHP