how to search array of checkboxes

Im building a search form but how do you search a array of checkboxes?

here is my html form

<form method="get">
<label>
   <input type="checkbox" name="material[]" value="metal">metal
</label>
<label>
   <input type="checkbox" name="material[]" value="metal">Plastic
</label>
<label>
   <input type="checkbox" name="material[]" value="metal">Carbom
</label>
<input type="text" name="keyword">
<input type="submit" value="search">
</form>

and the php so far is. So how can i search the material for each checked.

<?php
if(isset($_GET['keyword'])){
$keyword = $_GET['keyword'];
// $material = $_GET['material'];
// $Search->search($keyword);
}
?>