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