判斷php變數是否定義，是否為空白

isset() 【1】

Returns TRUE if var
exists and has value other
than NULL, FALSE otherwise.

輸入可以是多個變數，只有所有的變數為真的時候，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)

 

輸入只能是一個變數

 

is_null() 【3】

Returns TRUE if var is
null" href="http://www.php.net/manual/en/language.types.null.php">null , FALSE
otherwise.

 

 

 

   A variable is considered to be null" href="http://www.php.net/manual/en/language.types.null.php">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 函數對變數 $x 進行比較

運算式	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

 

（上表沒有找到原始來源，誰知道請告訴我）

 

如果變數是一個object該如何呢？

運算式	gettype()	empty()	is_null()	isset()	if($x) Boolean
$x = new object()	object	FALSE	FALSE	TRUE	TRUE

 

參考：

【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