php获取sitemap里面指定数量的网址URL并且保存到本地的TXT文本里面

[u0110baidu]

例如获取这个页面的http://localhost/sitemap.xml里面的内容如下所示

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>http://localhost/</loc>
<lastmod>2015-08-22T11:04:37+00:00</lastmod>
<changefreq>always</changefreq>
<priority>1.0</priority>
</url>
<url>
<loc>http://localhost/?post=362</loc>
<lastmod>2015-08-22T10:14:51+00:00</lastmod>
<changefreq>weekly</changefreq>
<priority>0.8</priority>
</url>
<url>
<loc>http://localhost/?post=361</loc>
<lastmod>2015-08-22T08:01:12+00:00</lastmod>
<changefreq>weekly</changefreq>
<priority>0.8</priority>
</url>

.............
我想要获取里面的<loc></loc>之间的URL(最好是可以设置获取URL的数量或者说顺序) --- http://localhost/?post=361 并且保存到本地的TXT文本里面(一条URL一行),php代码该如何操作 ?PS:试了一些截取方法没有成功,刚刚学习php..还希望大家指导一下,谢谢,尽量说得详细一点,非常谢谢.*_*

[xuzuning]

$s =<<< XML
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
<url>
<loc>http://localhost/</loc>
<lastmod>2015-08-22T11:04:37+00:00</lastmod>
<changefreq>always</changefreq>
<priority>1.0</priority>
</url>
<url>
<loc>http://localhost/?post=362</loc>
<lastmod>2015-08-22T10:14:51+00:00</lastmod>
<changefreq>weekly</changefreq>
<priority>0.8</priority>
</url>
<url>
<loc>http://localhost/?post=361</loc>
<lastmod>2015-08-22T08:01:12+00:00</lastmod>
<changefreq>weekly</changefreq>
<priority>0.8</priority>
</url>
</urlset>
XML;

$x = simplexml_load_string($s);
print_r($x);

SimpleXMLElement Object
(
    [url] => Array
        (
            [0] => SimpleXMLElement Object
                (
                    [loc] => http://localhost/
                    [lastmod] => 2015-08-22T11:04:37+00:00
                    [changefreq] => always
                    [priority] => 1.0
                )

            [1] => SimpleXMLElement Object
                (
                    [loc] => http://localhost/?post=362
                    [lastmod] => 2015-08-22T10:14:51+00:00
                    [changefreq] => weekly
                    [priority] => 0.8
                )

            [2] => SimpleXMLElement Object
                (
                    [loc] => http://localhost/?post=361
                    [lastmod] => 2015-08-22T08:01:12+00:00
                    [changefreq] => weekly
                    [priority] => 0.8
                )

        )

)

可知

foreach($x->url as $v) $r[] = $v->loc;
file_put_contents('文件.txt', join(PHP_EOL, $r));