The PHP Documentation Group has now added the Appendix K. PHP type comparison tables to the official PHP manual .

Empty() & Co, Special Values

?	gettype()	is_null()	isSet()	(bool)
$x = "";	string	false	true	false
$x = null;	NULL	true	false	false
var $x; (not set)	NULL	true	false	false
$x = array();	array	false	true	false
$x = false;	boolean	false	true	false
$x = 15;	integer	false	true	true
$x = 1;	integer	false	true	true
$x = 0;	integer	false	true	false
$x = -1;	integer	false	true	true
$x = "15";	string	false	true	true
$x = "1";	string	false	true	true
$x = "0";	string	false	true	false
$x = "-1";	string	false	true	true
$x = "foo";	string	false	true	true
$x = "true";	string	false	true	true
$x = "false";	string	false	true	true

Note: empty() and isSet() don't give a warning when the variable has not been defined before.