Pressing "Enter" to submit form

[lintz]

I've got a search form that users enter a stock code then click search and the results are displayed. My problem is a lot of users enter the stock code then hit "Enter" but with my checking (below) is doesn't think the search button has been clicked.

How can I capture "Enter" being pressed so I can perform the required search?

//Check if form has been submitted
if (!isset($_POST['submit']) ) {
//display search form
}

else {
//Show search results
}