PHP Notice: Undefined Index

[code?]

Okay, I keep getting this error when someone accesses the file.

PHP Notice:  Undefined index:  Ex in C:\\Server\\172.16.1.35\\WhosOnline.php on line (2, 3, and 33)

I noticed that in every line, there is something like this.

if ($_GET["Ex"] == "false") echo "</pre>"; 

Now, sometimes, there will not be a ?Ex=false, so I made it to this.

if ($_GET["Ex"])
{
    if ($_GET["Ex"] == "false") echo "</pre>";
} 

I still get the error that the index is undefined, and this snippet is not working how I thought it would, so how do I check if it is undefined, so it will stop giving me that error.