php isset emtpy

isset empty 参数可以是未定义的变量，不会产生警告。

isset 参数只能是变量，不能是数据。

empty($arr[x]) x是未定义元素，也不会产生警告，当$aa = $arr[x]就会产生警告了。

 1 <?php
 2 
 3 if(0 == null){
 4     echo "0 == null".PHP_EOL;
 5 }else{
 6     echo "0 != null".PHP_EOL;
 7 }
 8 
 9 if('' == null){
10     echo "'' == null".PHP_EOL;
11 }else{
12     echo "'' != null".PHP_EOL;
13 }
14 
15 if('0' == null){
16     echo "'0' == null".PHP_EOL;
17 }
18 else{
19     echo "'0' != null".PHP_EOL;
20 }
21 
22 if('0' == false){
23     echo "'0' == false".PHP_EOL;
24 }
25 else{
26     echo "'0' != false".PHP_EOL;
27 }
28 
29 if(false == null){
30     echo "false == null".PHP_EOL;
31 }else{
32     echo "false != null".PHP_EOL;
33 }
34 
35 if('0'){
36     echo "'0' is true".PHP_EOL;
37 }else{
38     echo "!'0' is true".PHP_EOL;
39 }
40 
41 $a = [null,null,1];
42 if(isset($a[0])){
43     echo '$a[0] isset true'.PHP_EOL;
44 }else{
45     echo '$a[0] isset false'.PHP_EOL;
46 }
47 if(isset($a[2])){
48     echo '$a[2] isset true'.PHP_EOL;
49 }else{
50     echo '$a[2] isset false'.PHP_EOL;
51 }
52 if(isset($a[10])){
53     echo '$a[10] isset true'.PHP_EOL;
54 }else{
55     echo '$a[10] isset false'.PHP_EOL;
56 }

empty(), is_null(), isset()(区别)