boolean isset
(mixed var)
Returns TRUE if var exists; FALSE otherwise.
If a variable has been unset with unset(), it will no longer be isset(). isset() will return FALSE if testing a variable that has been set to NULL. Also note that a NULL byte ("\0") is not equivalent to the PHP NULL constant.
$a = "test"; echo isset ($a); // TRUE unset ($a); echo isset ($a); // FALSE $foo = NULL; print isset ($foo); // FALSE |