verifying if checkbox is checked in php

I want to verify if my checkbox is checked in php and if is it i want to echo "Hello word". There is my html code :

<input type="checkbox" name="checkbox1"> 4K </input>

php :

 <?php
if (isset($_POST['checkbox1'])) {
    echo "Hello world!";
}
?>

But it don't work and i really don't now how to do this. Can someone please help me ?