In where shall I use isset() and !empty()

isset vs. !empty

FTA:

"isset() checks if a variable has a value including (False, 0 or empty string), but not NULL. Returns TRUE if var exists; FALSE otherwise.

On the other hand the empty() function checks if the variable has an empty value empty string, 0, NULL or False. Returns FALSE if var has a non-empty and non-zero value."

In the most general way :

• isset tests if a variable (or an element of an array, or a property of an object) exists (and is not null)
• empty tests if a variable (...) contains some non-empty data.

To answer question 1 :

$str = '';
var_dump(isset($str));

gives

boolean true

Because the variable $str exists.

And question 2 :

You should use isset to determine whether a variable exists ; for instance, if you are getting some data as an array, you might need to check if a key isset in that array.
Think about $_GET / $_POST, for instance.

Now, to work on its value, when you know there is such a value : that is the job of empty.