simplexml_load_string() 不会读取带有“soap:"的soap响应；在标签中

I know this may be a newbie question, but please humor me. When reading an xml string with "soap:" in the tags, simplexml_load_string() will not read in the xml.

给定这个脚本:

#!/usr/bin/php
<?php

$s='
    <soap:Envelope xmlns:soap="http://www.w3.org/2003/05/soap-envelope">
        <soap:Header>
        <context xmlns="urn:zimbra"/>
        </soap:Header>
        <soap:Body>
            <AuthResponse xmlns="urn:zimbraAdmin">
                <authToken>somevalue</authToken>
                <lifetime>123124123</lifetime>
                <a n="zimbraIsDomainAdminAccount">false</a>
            </AuthResponse>
        </soap:Body>
    </soap:Envelope>';


print_r(simplexml_load_string($s));
echo "

";
print_r(simplexml_load_string(str_ireplace("soap:", "", $s)));
?>

我得到这个输出:

jesse@jesse-debian:~/code/zmsoap$ ./xmltest.php 
SimpleXMLElement Object
(
)

SimpleXMLElement Object
(
    [Header] => SimpleXMLElement Object
        (
            [context] => SimpleXMLElement Object
                (
                )

        )

    [Body] => SimpleXMLElement Object
        (
            [AuthResponse] => SimpleXMLElement Object
                (
                    [authToken] => somevalue
                    [lifetime] => 123124123
                    [a] => false
                )

        )

)
jesse@jesse-debian:~/code/zmsoap$

我只是很好奇为什么会发生这种情况，以及是否有比进行字符串替换更合适的方法来解决该问题.