check if variable empty

If you want to test whether a variable is really NULL, use the identity operator:

$user_id === NULL  // FALSE == NULL is true, FALSE === NULL is falseis_null($user_id)

If you want to check whether a variable is not set:

!isset($user_id)

Or if the variable is not empty, an empty string, zero, ..:

empty($user_id)

If you want to test whether a variable is not an empty string, ! will also be sufficient:

!$user_id

You can check if it's not set (or empty) in a number of ways.

if (!$var){ }

Or:

if ($var === null){ } // This checks if the variable, by type, IS null.

Or:

if (empty($var)){ }

You can check if it's declared with:

if (!isset($var)){ }

Take note that PHP interprets 0 (integer) and "" (empty string) and false as "empty" - and dispite being different types, these specific values are by PHP considered the same. It doesn't matter if $var is never set/declared or if it's declared as $var = 0 or $var = "". So often you compare by using the === operator which compares with respect to data type. If $var is 0 (integer), $var == "" or $var == false will validate, but $var === "" or $var === false will not.

empty() is a little shorter, as an alternative to checking !$user_id as suggested elsewhere:

if (empty($user_id) || empty($user_name) || empty($user_logged)) {}