判断php变量是否定义，是否为空

isset() 【1】
Returns TRUE if var
exists and has value other
than NULL, FALSE otherwise.
输入可以是多个变量，只有所有的变量为真的时候，返回真
empty()【2】
Returns FALSE if var
has a non-empty
and non-zero value.

The following things are considered to be empty:

"" (an empty string)
0 (0 as an integer)
"0" (0 as a string)
NULL
FALSE
array() (an empty array)
var $var; (a variable declared, but without a
value in a class)
输入只能是一个变量
is_null() 【3】
Returns TRUE if var is
null , FALSE
otherwise.
   A variable is considered to be null if:

it has been assigned the constant NULL.

it has not been set to any value yet.

it has been unset().    
使用 PHP 函数对变量 $x 进行比较
表达式gettype()empty()is_null()isset()if($x) Boolean
$x = "";
string
TRUE
FALSE
TRUE
FALSE
$x = null;
NULL
TRUE
TRUE
FALSE
FALSE
var $x;
NULL
TRUE
TRUE
FALSE
FALSE
$x is undefined
NULL
TRUE
TRUE
FALSE
FALSE
$x = array();
array
TRUE
FALSE
TRUE
FALSE
$x = false;
boolean
TRUE
FALSE
TRUE
FALSE
$x = true;
boolean
FALSE
FALSE
TRUE
TRUE
$x = 1;
integer
FALSE
FALSE
TRUE
TRUE
$x = 42;
integer
FALSE
FALSE
TRUE
TRUE
$x = 0;
integer
TRUE
FALSE
TRUE
FALSE
$x = -1;
integer
FALSE
FALSE
TRUE
TRUE
$x = "1";
string
FALSE
FALSE
TRUE
TRUE
$x = "0";
string
TRUE
FALSE
TRUE
FALSE
$x = "-1";
string
FALSE
FALSE
TRUE
TRUE
$x = "php";
string
FALSE
FALSE
TRUE
TRUE
$x = "true";
string
FALSE
FALSE
TRUE
TRUE
$x = "false";
string
FALSE
FALSE
TRUE
TRUE
（上表没有找到原始来源，谁知道请告诉我）
如果变量是一个object该如何呢？
表达式gettype()empty()is_null()isset()if($x) Boolean
$x = new object() 
object
FALSE
FALSE
TRUE
TRUE
参考：
【1】http://php.net/manual/en/function.isset.php
【2】http://www.php.net/manual/en/function.empty.php
【3】http://www.php.net/manual/en/function.is-null.php