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HomeBackend DevelopmentPHP Tutorial第一天弄PHP,崩溃了,这都报错?

第一天搞PHP,崩溃了,这都报错???

<br />if($_GET['all']=='yes')<br />{<br />    echo "hi";  <br />}<br />

报错:Notice: Undefined index: all in C:\Users\Administrator\Downloads\PHPnow-1.5.6.4237493736\htdocs\oa\manage.php on line 71

小弟实在搞不清,一个等号  两个等号   到底有什么区别。
另外,如果改成一个等号,不报错了,但是这条语句也没有用了,url上面有all=yes 会出现hi, url上面没有all=yes还是会出现hi.

------解决方案--------------------
if(isset($_GET['all']) && $_GET['all']=='yes')
{
    echo "hi";  
}
------解决方案--------------------
$_GET['all'] 未定义,不是错误,只是提示。

可以用isset() 方法判断是否已定义再取值。

例如:
<br />if(isset($_GET['all']) && $_GET['all']=='yes')<br />{<br />    echo "hi";  <br />}<br />

------解决方案--------------------
一个等号 是赋值
 两个等号 才是等于

正确的写法应该先用isset判断一下。

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