PHP里面，有众多的函数检查一个变量是否存在，或者是否为真，或者是否为空。例如： empty, is_null, isSet, == null等等，你知道他们之间的细节差异吗？表中的T就是True, F就是False
原文地址: http://www.blueshoes.org/en/developer/php_cheat_sheet/
检查变量的函数
|
gettype() |
empty() |
is_null() |
isSet() |
(bool) |
| $x = “”; |
string |
T |
F |
T |
F |
| $x = null; |
NULL |
T |
T |
F |
F |
| var $x; (not set) |
NULL |
T |
T |
F |
F |
| $x = array(); |
array |
T |
F |
T |
F |
| $x = false; |
boolean |
T |
F |
T |
F |
| $x = 15; |
integer |
F |
F |
T |
T |
| $x = 1; |
integer |
F |
F |
T |
T |
| $x = 0; |
integer |
T |
F |
T |
F |
| $x = -1; |
integer |
F |
F |
T |
T |
| $x = “15″; |
string |
F |
F |
T |
T |
| $x = “1″; |
string |
F |
F |
T |
T |
| $x = “0″; |
string |
T |
F |
T |
F |
| $x = “-1″; |
string |
F |
F |
T |
T |
| $x = “foo”; |
string |
F |
F |
T |
T |
| $x = “true”; |
string |
F |
F |
T |
T |
| $x = “false”; |
string |
F |
F |
T |
T |
注意： empty() 和isSet() 在传入一个未定义变量的时候，报warning的。
用==比较
|
true |
false |
1 |
0 |
-1 |
“1″ |
“0″ |
“-1″ |
“1.3″ |
1.3 |
array() |
“foo” |
“” |
| true |
T |
F |
T |
F |
T |
T |
F |
T |
T |
T |
F |
T |
F |
| false |
F |
T |
F |
T |
F |
F |
T |
F |
F |
F |
T |
F |
T |
| 1 |
T |
F |
T |
F |
F |
T |
F |
F |
F |
F |
F |
F |
F |
| 0 |
F |
T |
F |
T |
F |
F |
T |
F |
F |
F |
F |
T |
T |
| -1 |
T |
F |
F |
F |
T |
F |
F |
T |
F |
F |
F |
F |
F |
| “1″ |
T |
F |
T |
F |
F |
T |
F |
F |
F |
F |
F |
F |
F |
| “0″ |
F |
T |
F |
T |
F |
F |
T |
F |
F |
F |
F |
F |
F |
| “-1″ |
T |
F |
F |
F |
T |
F |
F |
T |
F |
F |
F |
F |
F |
| “1.3″ |
T |
F |
F |
F |
F |
F |
F |
F |
T |
T |
F |
F |
F |
| 1.3 |
T |
F |
F |
F |
F |
F |
F |
F |
T |
T |
F |
F |
F |
| array() |
F |
T |
F |
F |
F |
F |
F |
F |
F |
F |
T |
F |
F |
| “foo” |
T |
F |
F |
T |
F |
F |
F |
F |
F |
F |
F |
T |
F |
| “” |
F |
T |
F |
T |
F |
F |
F |
F |
F |
F |
F |
F |
T |
用===比较
|
true |
false |
1 |
0 |
-1 |
“1″ |
“0″ |
“-1″ |
“1.3″ |
1.3 |
array() |
“foo” |
“” |
| true |
T |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
| false |
F |
T |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
| 1 |
F |
F |
T |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
| 0 |
F |
F |
F |
T |
F |
F |
F |
F |
F |
F |
F |
F |
F |
| -1 |
F |
F |
F |
F |
T |
F |
F |
F |
F |
F |
F |
F |
F |
| “1″ |
F |
F |
F |
F |
F |
T |
F |
F |
F |
F |
F |
F |
F |
| “0″ |
F |
F |
F |
F |
F |
F |
T |
F |
F |
F |
F |
F |
F |
| “-1″ |
F |
F |
F |
F |
F |
F |
F |
T |
F |
F |
F |
F |
F |
| “1.3″ |
F |
F |
F |
F |
F |
F |
F |
F |
T |
F |
F |
F |
F |
| 1.3 |
F |
F |
F |
F |
F |
F |
F |
F |
F |
T |
F |
F |
F |
| array() |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
T |
F |
F |
| “foo” |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
T |
F |
| “” |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
F |
T |