The simplest method to POST a single on or off value (PHP)

This should be SO simple so I must be doing something wrong but I have a single checkbox to check whether a value is on or off

<input type="checkbox" name="highlight">

and when it is posted, if it is checked I get the value 'on' but if it is not checked I get an error

Notice: Undefined index: highlight 

Now I have tried testing whether it is empty or !isset but I would like to know if this is the correct method for passing a boolean value, it seems a bit convoluted.