PHP If Else Statement T MAke Sure a Check Box is Checked

[scheols]

<input type=checkbox name=sentence1 value=1>

alright say i have some TOS rules on registration page how can i make it so u have to press the checkbox to register else returns a Message Saying CLick Back and blahblahblah.

[thorpe]

if (!isset($_POST['sentence1']) || empty($_POST['sentence1'])) {
  echo "CLick Back and blahblahblah.";
}