判断php变量是不是定义，是否为空

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`
$x = “php”;	string	`本文@来#源gaodai$ma#com搞$$代码网搞代gaodaima码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

http://blog.gaodaima.com/autofei/archive/2010/05/24/5619004.aspx

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