php解析xml字符串

<?php
$content = <<<XML
<?xml version="1.0" encoding="UTF-8"?>
<test>
    <global_setting>
        <ping_protocol>HTTP</ping_protocol>
        <ping_port>80</ping_port>
        <ping_path>/index.html</ping_path>
        <response_timeout>5000</response_timeout>
        <health_check_interval>3000</health_check_interval>
        <unhealthy_threshold>2</unhealthy_threshold>
        <healthy_threshold>3</healthy_threshold>
    </global_setting>
    <instances>
        <instance ip="192.168.234.121"/>
        <instance ip="192.168.234.28"/>
    </instances>
</test>
XML;

$test = new SimpleXMLElement($content);  

//获得ping_protocol的值
$ping_protocol = $test->global_setting->ping_protocol;
echo "ping_protocol : $ping_protocol \n";  

//打印出所有instance的IP
foreach ( $test->instances->instance as $instance) {
    echo "IP: {$instance[‘ip‘]} \n" ;
}  

//经过我的测试,发现试用var_dump之类的似乎不能有效输出值,用echo比较顺利,还有就是上面的那个xml的例子可以去掉<?xml version="1.0" encoding="UTF-8"?>  也可以去掉头尾///的<<<xml,然后当做普通字符串那样对待,但是没有测试中文等