""、0、null三个值用var_dump打印结果如下：

string(0) "" int(0) NULL

用== 对其进行比较：

if("" == 0){    echo "1
";}else{    echo "2
";}if("" == null){    echo "3
";}else{    echo "4
";}if(null == 0){    echo "5
";}else{    echo "6
";}

结果为：

1

3

5

===比较

if("" === 0){    echo "1
";}else{    echo "2
";}if("" === null){    echo "3
";}else{    echo "4
";}if(null === 0){    echo "5
";}else{    echo "6
";}

结果为：

2

4

6

==比较的是值，===比较的不止是值还有类型