简单的html dom ahref第一个链接

我需要一些帮助，html是这样的：

<div class="index_item index_item_ie"><a href="/watch-2757880-United-We- Fall-online-free" title="Watch United We Fall (2014)"><img src="//images.primewire.ag/thumbs/2757880_United_We_Fall_2014.jpg" border="0" width="150" height="225" alt="Watch United We Fall"><h2>United We Fall (2014)</h2></a><div class="index_ratings"><div id="unit_long2757880"><ul style="width: 100px;" class="unit-rating"><li style="width: 100px;" class="current-rating">Current rating.</li><li class="r1-unit"></li><li class="r2-unit"></li><li class="r3-unit"></li><li class="r4-unit"></li><li class="r5-unit"></li></ul></div></div>
<div class="item_categories"><a href="/?genre=Comedy">Comedy</a> </div></div>

我试图只获取第一个链接/watch-2757880-United-We-Fall-online-free

但返回了所有链接

$ret =  $html->find('div.index_item');
foreach($ret as $elements) {
    foreach($elements->find('a') as $link) {
        $url = $link->href;
        echo $url;
    }
}

如何只获取第一个链接？/watch-2757880-United-We-Fall-online-free