isset

Description

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 
      

See also empty() and unset().