codeigniter and youtube api - xml woes

我正在研究一个使用codeigniter和youtube api的项目，我请求一个视频，我得到了一些XML，我的问题是我不知道该怎么处理所说的XML，或者更重要的是如何解析它，这样我就可以得到我想要的值，我得到它的XML，

<?xml version='1.0' encoding='UTF-8'?>
<entry xmlns='http://www.w3.org/2005/Atom' xmlns:media='http://search.yahoo.com/mrss/' xmlns:gd='http://schemas.google.com/g/2005'
xmlns:yt='http://gdata.youtube.com/schemas/2007'>
    <id>http://gdata.youtube.com/feeds/api/videos/x33JD3-hXHc</id>
    <published>2010-11-29T21:28:22.000Z</published>
    <updated>2011-03-14T12:01:51.000Z</updated>
    <category scheme='http://schemas.google.com/g/2005#kind' term='http://gdata.youtube.com/schemas/2007#video'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/categories.cat' term='Travel' label='Travel &amp; Events'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='Barney'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='Harwood'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='Interview'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='children&apos;s'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='bafta'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='awards'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='2010'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='red'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='carpet'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='host'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='tv'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='television'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='show'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='presenter'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='prank'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='patrol'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='the'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='sorcerer&apos;s'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='apprentice'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='bbc'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='cbbc'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='bear'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='behaving'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='badly'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='nev'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='smile'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='totally'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='doctor'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='who'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='basil&apos;s'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='swap'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='shop'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='basil'/>
    <category scheme='http://gdata.youtube.com/schemas/2007/keywords.cat' term='brush'/>
    <title type='text'>Barney Harwood Interview</title>
    <content type='text'>Barney Harwood Interview - We talk to children's TV presenter Barney Harwood on the red carept, ahead of his duties as host for the Children's BAFTA Awards 2010 in London. Presented by Michael Kurn Camera and Post by Russell Nelson</content>
    <link rel='alternate' type='text/html' href='http://www.youtube.com/watch?v=x33JD3-hXHc&amp;feature=youtube_gdata'/>
    <link rel='http://gdata.youtube.com/schemas/2007#video.responses' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/videos/x33JD3-hXHc/responses'/>
    <link rel='http://gdata.youtube.com/schemas/2007#video.ratings' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/videos/x33JD3-hXHc/ratings'/>
    <link rel='http://gdata.youtube.com/schemas/2007#video.complaints' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/videos/x33JD3-hXHc/complaints'/>
    <link rel='http://gdata.youtube.com/schemas/2007#video.related' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/videos/x33JD3-hXHc/related'/>
    <link rel='http://gdata.youtube.com/schemas/2007#mobile' type='text/html' href='http://m.youtube.com/details?v=x33JD3-hXHc'/>
    <link rel='self' type='application/atom+xml' href='http://gdata.youtube.com/feeds/api/videos/x33JD3-hXHc'/>
    <author>
        <name>LeicesterSquareTV</name>
        <uri>http://gdata.youtube.com/feeds/api/users/leicestersquaretv</uri>
    </author>
    <gd:comments>
        <gd:feedLink href='http://gdata.youtube.com/feeds/api/videos/x33JD3-hXHc/comments' countHint='5'/>
    </gd:comments>
    <yt:hd/>
    <media:group>
        <media:category label='Travel &amp; Events' scheme='http://gdata.youtube.com/schemas/2007/categories.cat'>Travel</media:category>
        <media:content url='http://www.youtube.com/v/x33JD3-hXHc?version=3&amp;f=videos&amp;d=Ac2cvuTKQbM_TEZ_NAvfWNEO88HsQjpE1a8d1GxQnGDm&amp;app=youtube_gdata' type='application/x-shockwave-flash' medium='video' isDefault='true' expression='full' duration='150' yt:format='5'/>
        <media:content url='rtsp://v4.cache7.c.youtube.com/CkULENy73wIaPAl3XKF_D8l9xxMYDSANFEgGUgZ2aWRlb3NyIQHNnL7kykGzP0xGfzQL31jRDvPB7EI6RNWvHdRsUJxg5gw=/0/0/0/video.3gp' type='video/3gpp' medium='video' expression='full' duration='150' yt:format='1'/>
        <media:content url='rtsp://v2.cache5.c.youtube.com/CkULENy73wIaPAl3XKF_D8l9xxMYESARFEgGUgZ2aWRlb3NyIQHNnL7kykGzP0xGfzQL31jRDvPB7EI6RNWvHdRsUJxg5gw=/0/0/0/video.3gp' type='video/3gpp' medium='video' expression='full' duration='150' yt:format='6'/>
        <media:description type='plain'>Barney Harwood Interview -We talk to children's TV presenter Barney Harwood on the red carept, ahead of his duties as host for the Children's BAFTA Awards 2010 in London. Presented by Michael Kurn
        Camera and Post by Russell Nelson
        </media:description>
        <media:keywords>Barney, Harwood, Interview, children's, bafta, awards, 2010, red, carpet, host, tv, television, show, presenter, prank, patrol, the, sorcerer's, apprentice, bbc, cbbc, bear, behaving, badly, nev, smile, totally, doctor, who, basil's, swap, shop, basil, brush
        </media:keywords>
        <media:player url='http://www.youtube.com/watch?v=x33JD3-hXHc&amp;feature=youtube_gdata_player'/>
        <media:thumbnail url='http://i.ytimg.com/vi/x33JD3-hXHc/0.jpg' height='360' width='480' time='00:01:15'/>
        <media:thumbnail url='http://i.ytimg.com/vi/x33JD3-hXHc/1.jpg' height='90' width='120' time='00:00:37.500'/>
        <media:thumbnail url='http://i.ytimg.com/vi/x33JD3-hXHc/2.jpg' height='90' width='120' time='00:01:15'/>
        <media:thumbnail url='http://i.ytimg.com/vi/x33JD3-hXHc/3.jpg' height='90' width='120' time='00:01:52.500'/>
        <media:title type='plain'>Barney Harwood Interview</media:title><yt:duration seconds='150'/>
    </media:group>
    <gd:rating average='5.0' max='5' min='1' numRaters='1' rel='http://schemas.google.com/g/2005#overall'/>
    <yt:statistics favoriteCount='0' viewCount='1141'/>
</entry>

具体来说，我如何获得缩略图?