判断php变量是否定义，是否为空
isset() 【1】Returns TRUE if var exists and has value otherthan 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 varhas a non-emptyand 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 $var; (a variable declared, but without a value in a class)
输入只能是一个变量
is_null() 【3】
Returns TRUE if var isnull , FALSEotherwise.
A variable is considered to benullif:
[*]it has been assigned the constant NULL.
[*]it has not been set to any value yet.
[*]it has been unset().
使用 PHP 函数对变量$x进行比较表达式gettype()empty()is_null()isset()if($x) Boolean$x = "";stringTRUEFALSETRUEFALSE$x = null;NULLTRUETRUEFALSEFALSEvar $x;NULLTRUETRUEFALSEFALSE$xis undefinedNULLTRUETRUEFALSEFALSE$x = array();arrayTRUEFALSETRUEFALSE$x = false;booleanTRUEFALSETRUEFALSE$x = true;booleanFALSEFALSETRUETRUE$x = 1;integerFALSEFALSETRUETRUE$x = 42;integerFALSEFALSETRUETRUE$x = 0;integerTRUEFALSETRUEFALSE$x = -1;integerFALSEFALSETRUETRUE$x = "1";stringFALSEFALSETRUETRUE$x = "0";stringTRUEFALSETRUEFALSE$x = "-1";stringFALSEFALSETRUETRUE$x = "php";stringFALSEFALSETRUETRUE$x = "true";stringFALSEFALSETRUETRUE$x = "false";stringFALSEFALSETRUETRUE （上表没有找到原始来源，谁知道请告诉我）
如果变量是一个object该如何呢？
表达式gettype()empty()is_null()isset()if($x) Boolean$x = new object()objectFALSEFALSETRUETRUE 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
页:
[1]