check for empty values:

if($_GET['email'] == ""){
header("Location: index.html");
exit;
}

Forms -

Radio button choices -

<p>
<input type="radio" name="color" value="red" />
red</p>
<p>
<input type="radio" name="color" value="green" />
green</p>
<p>
<input type="radio" name="color" value="blue" />
blue
</p>
<p>
<input type="radio" name="color" value="yellow" />
yellow
</p>

drop down menu choice-

<p>How hot do you like your hot sauce?
<select name="How do you like your hot sauce?" id="How do you like your hot sauce?">
<option>Not too hot</option>
<option>a little hot</option>
<option>ok, turn it up a bit</option>
<option>Let me have it</option>
<option>nuclear level</option>
</select>
</p>