how to see if all checkboxes are empty in php

how to see if all checkboxes are empty in php

In PHP, how do I see if all checkboxes are empty? This is an excerpt from
my code: Basically, I want to use an "if" and "else" code that is
specifically for when all my checkboxes are unchecked.
$titlebox=isset($_GET['title']) ? "checked='checked'" : '';
$authorbox=isset($_GET['author']) ? "checked='checked'" : '';
// keeps checkboxes checked after submit
<form action="" method="get">
<input type="text" name="search">
<input type="submit" value="Search">
<input type="checkbox" name="title" '.$titlebox.' >
<label for="title">Title</label>
<input type="checkbox" name="author" '.$authorbox.' >
<label for="author">Author</label>
</form>
// form
if (isset($_GET['title']))
{ $searchbooktitle = $bookfieldtitle;
}
else
{ $searchbooktitle = NULL;
};
// this is how I get individual checkboxes.
The last part shows how I get individual checkboxes, but how do I get all
the checkboxes and see if they are empty? I'd like to use a specific "if"
and "else" code that will strictly apply only if all checkboxes are
unchecked.
The answer might be simple but I'm new at this and confused. Please help!