You can name your checkboxes something like
<input type='checkbox' id='product_01' name='products[]' value='1'>
<input type='checkbox' id='product_02' name='products[]' value='2'>
<input type='checkbox' id='product_03' name='products[]' value='3'>
Then, in your PHP code you can get all of the selected values in an array like this
$selected_products = $_POST['products'];
foreach($selected_products as $product_id) {
echo "You selected product: $product_id";
}
All of the products that the user selects will be in the $selected_products array.