[php] PHP Undefined Index

This is going to sound really stupid, but I cannot figure out why I am getting this error.

I have created a selection box, named "query_age" in my html form:

<form method="get" action="user_list.php">
<select name="query_age">
  <option value="">Doesn't matter</option>
  <option value="between 18 and 30">18 - 30</option>
  <option value="between 31 and 40">31 - 40</option>
  <option value="between 41 and 50">41 - 50</option>
  <option value="between 51 and 60">51 - 60</option>
  <option value="between 61 and 70">61 - 70</option>
  <option value="between 71 and 80">71 - 80</option>
  <option value="between 81 and 90">81 - 90</option>
  <option value="> 90">Older than 90</option>
</select>

In the corresponding php form, I have:

$query_age = $_GET['query_age'];

When I run the page, I get this error:

Notice: Undefined index: query_age in index.php on line 19

I don't understand why this is happening, and I'd love to know how to make it go away.

The checking of the presence of the member before assigning it is, in my opinion, quite ugly.

Kohana has a useful function to make selecting parameters simple.

You can make your own like so...

function arrayGet($array, $key, $default = NULL)
{
    return isset($array[$key]) ? $array[$key] : $default;
}

And then do something like...

$page = arrayGet($_GET, 'p', 1);

The first time you run the page, the query_age index doesn't exist because it hasn't been sent over from the form.

When you submit the form it will then exist, and it won't complain about it.

#so change
$_GET['query_age'];
#to:
(!empty($_GET['query_age']) ? $_GET['query_age'] : null);

if you use isset like the answer posted already by singles, just make sure there is a bracket at the end like so:
$query_age = (isset($_GET['query_age']) ? $_GET['query_age'] : null);