So many differences and little similarities between ISSET and EMPTY in php.

ISSET

Isset returns TRUE if the variable exists and has a value other than NULL.

<?php 

$message = null;
 if(isset($message){
 return "message is set";
}
 return "message is not set";
//returns "message is not set";

 $message = "";
 if(isset($message){
 return "message is set";
}
 return "message is not set";
//returns "message is set";

 $array = ["foo" => "bar", "bar" => "foo"];
 if(isset($array["food"]){
 return "food is set";
}
 return "food is not set";
//returns "food is not set";

EMPTY

Empty checks to see if a variable is empty. such as: "" (an empty string), 0 (integer), 0.0 (float)`, "0" (string), null, false

$message = null;
 if(!empty($message){
 return "message is not empty";
}
 return "message is empty";
//returns "message is empty";

 $message = "";
 if(empty($message){
  return "message is empty";
}
 return "message is not empty";
//returns "message is empty";

 $array = ["foo" => "bar", "bar" => "foo"];
 if(empty($array["food"]){
 return "food is empty";
}
 return "food is not empty";
//returns "food is empty";

You can see the difference between Isset and Empty... if a variable is set to any value apart from null, Isset returns true including if the variable is set to false, but Empty covers everything including empty string and false