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 class="clear"> <a href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL2suaHRtbD90PTI=" target="_blank" rel="nofollow"><img onerror="this.src='/statics/superweb999/images/image_error.jpg'"src="/default/index/img?u=aHR0cHM6Ly9pbWcucGhwLmNuL3VwbG9hZC9jb3Vyc2UvMDAwLzAwMC8wNzEvNjIwYzYwNGY3NTI1MTk5NS5wbmc=" style="margin-top: 30px;" alt="帮小弟我解释一句代码" title="帮小弟我解释一句代码"></a><p style="float:right;font-size:13px;color:#999;"><span class="red">声明：</span>本文内容由网友自发贡献，版权归原作者所有，本站不承担相应法律责任。如您发现有涉嫌抄袭侵权的内容，请联系[e-mail]核实处理。</p> </div> <p class="article-relative-header">相关文章</p> <p class="article-relative-header">相关视频</p> <hr class="layui-clear"> <ul class="article-relative-ul"> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL3BocC13ZWl6aWppYW9jaGVuZy00ODg4ODkuaHRtbA==" target="_blank" rel="nofollow">详解win10下PHP的安装配置（以php5.6为...</a> </li> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL3BocC13ZWl6aWppYW9jaGVuZy00ODg4MzUuaHRtbA==" target="_blank" rel="nofollow">php Swoole实现毫秒定时计划任务（详解）</a> </li> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL3BocC13ZWl6aWppYW9jaGVuZy00ODg4OTAuaHRtbA==" target="_blank" rel="nofollow">一文详解Windows和Linux环境下怎么安装配...</a> </li> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL3BocC13ZWl6aWppYW9jaGVuZy00ODg4NTQuaHRtbA==" target="_blank" rel="nofollow">【DTM】PHP协程客户端v0.1 beta版本发...</a> </li> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL3BocC13ZWl6aWppYW9jaGVuZy0yMjAwOTYuaHRtbA==" target="_blank" rel="nofollow"> 帮小弟我解释一句代码 </a> </li> </ul> <ul class="article-relative-ul"> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL2NvZGUvOTEyLmh0bWw=" target="_blank" title='.E[attr~="value"]选择器' rel="nofollow">.E[attr~="value"]选择器</a> </li> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL2NvZGUvOTE1Lmh0bWw=" target="_blank" title='E[attr^="value"]选择器' rel="nofollow">E[attr^="value"]选择器</a> </li> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL2NvZGUvOTE2Lmh0bWw=" target="_blank" title='E[attr$="value"]选择器' rel="nofollow">E[attr$="value"]选择器</a> </li> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL2NvZGUvOTE3Lmh0bWw=" target="_blank" title='E[attr*="value"]选择器' rel="nofollow">E[attr*="value"]选择器</a> </li> <li> <span class="layui-badge-dots"></span><a class="relevant" href="/default/index/url?u=aHR0cHM6Ly93d3cucGhwLmNuL2NvZGUvOTE4Lmh0bWw=" target="_blank" title='E[attr|="value"]选择器' rel="nofollow">E[attr|="value"]选择器</a> </li> </ul>