我用PHP产生了一个UTF-8 RSS feed xml string,字符串的开头是：<?xml version="1.0" encoding="utf-8"?><?xml-stylesheet type="text/xsl" href="rsspretty.xsl" version="1.0"?>。字符串的开头没有BOM字符。但用header("Content-Type: text/xml; charset=utf-8") 和echo显示时，串的开头被加入几个BOM， 得到 “The XML page cannot be displayed。Cannot view XML input using style sheet. Please correct the error and then click the Refresh button, or try again later. Invalid at the top level of the document“ 错误。请问应如何格式化显示UTF-8 RSS feed ？