PHP限制图片上传的格式

PHP限制图片上传的格式，如下代码：

<ol class="dp-xml">
<li class="alt"><span><span class="tag"><span class="tag-name">html</span><span class="tag">></span><span> </span></span></span></li>
<li><span><span class="tag"><span class="tag-name">head</span><span class="tag">></span><span> </span></span></span></li>
<li class="alt"><span><span class="tag"><span class="tag-name">meta</span><span> </span><span class="attribute">http-equiv</span><span>=</span><span class="attribute-value">"Content-Type"</span><span> </span><span class="attribute">content</span><span>=</span><span class="attribute-value">"text/html; charset=gb2312"</span><span class="tag">></span><span> </span></span></span></li>
<li><span><span class="tag"><span class="tag-name">title</span><span class="tag">></span><span>限制上传图片的格式 - www.cxybl.com</span><span class="tag"></span><span class="tag-name">title</span><span class="tag">></span><span> </span></span></span></li>
<li class="alt"><span><span class="tag"><span class="tag-name">style</span><span> </span><span class="attribute">type</span><span>=</span><span class="attribute-value">"text/css"</span><span class="tag">></span><span> </span></span></span></li>
<li><span></span></li>
</ol>