XML error at ampersand (&)
$string =htmlspecialchars($string,ENT_XML1);
is the most universal way to solve all encoding errors (IMHO better that write custom functions + there is no point to solve just &).
Credit: Put Wrikken's and joshweir's comment as answer to be more visible.
& in XML starts an entity. As you haven't defined an entity &WhateverIsAfterThat an error is thrown. You should escape it with &.
$string = str_replace('&', '&', $string);
How do I escape ampersands in XML
To escape the other reserved characters:
function xmlEscape($string) {
return str_replace(array('&', '<', '>', '\'', '"'), array('&', '<', '>', ''', '"'), $string);
}