PHP empty, Isset, isnull the difference

Empty

If the variable is a non-null or nonzero value, empty () returns FALSE. In other words, "", 0, "0″, NULL, FALSE, Array (), Var $var, undefined, and objects that do not have any properties will be considered empty, and if Var is null, it returns TRUE."

Isset

Returns TRUE if the variable exists (not null), otherwise FALSE (including undefined). The variable value is set to: null, and the variable is canceled after the return is also false;unset a variable. Note that isset for null value variables, special handling.

Is_null

Detects if the passed-in value "value, variable, expression" is null, only one variable is defined, and its value is NULL, it returns TRUE. All others return FALSE "an undefined variable is passed in with an error!" 】

Instance code:

function Response_data ($data) {        $this->output->set_header (' Content-type:application/json; charset=utf-8 ');        if (empty ($data)) {            $data = array ();        }        echo Json_encode ($data);    }

PHP empty, Isset, isnull the difference