帮小弟我解释一句代码

帮我解释一句代码
$str = (isset($_POST['source']) ? $_POST['source'] : '');
这句话怎么理解啊。其中的问号和冒号又分别起什么作用啊。麻烦
大神们指点下。我找资料没找到。
其中source来自于 <br><br /><font color='#e78608'>------解决方案--------------------</font><br />换成这样写，<br />$source=$_POST["source"];<br />$str=(isset($scource)?$scource:null);<br />如果$source有值，就取$source的值赋给$str，不然就赋值空给$str<br />还哪不明白？ <br /><font color='#e78608'>------解决方案--------------------</font><br /> a ? b : c 三元运算符<br />等价于<br />if(a) {<br /> return b;<br />}else {<br /> return c;<br />} </div> <div class=""> <ul class="m-news-opt fix"> <li class="opt-item"> <a href='/PHPjiqiao-113847.html' target='_blank'><p>< 上一篇</p><p class="ellipsis">YouhaveanerrorinyourSQLsyntax,该怎么解决</p></a> </li> <li class="opt-item ta-r"> <a href='/PHPjiqiao-113849.html' target='_blank'><p>下一篇 ></p><p class="ellipsis">这样的PHP正则表达式该如何写</p></a> </li> </ul> </div> </div> </div> <div class="g-title fix"> <h2 class="title-txt">人气教程排行</h2> </div> <div class="m-rank u-dashed mb40"> <ul> <li class="rank-item"> <a href="/PHPjiqiao-379253.html" title='php如何获取跳转前的url' class="item-name ellipsis" target="_blank"> <span 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