Undefined index: search in C:wampwwwsearch1 (2).php on line 4

This is my code:

$button = $_GET ['submit'];
$search = $_GET ['search']; 

if(!$button)
echo "you didn't submit a keyword!";
else

And I'm getting this error:

Undefined index: search in C:\wamp\www\search1 (2).php on line 4