PHP中empty()和isset()的区别介绍

二者共同点

二者区别

1、对于未设置的变量的判断

2、对于 "" (空字符串) 的判断

3、对于 0 (作为整数的0) 的判断

4、对于 0.0 (作为浮点数的0) 的判断

5、对于 "0" (作为字符串的0) 的判断

6、对于 NULL 的判断

7、对于 FALSE 的判断

8、对于 array() (一个空数组) 的判断

二者共同点

都可以判定一个变量是否为空；

都返回boolean类型，即true或false。

二者区别

empty() 函数用于检查一个变量是否为空。当一个变量并不存在，或者它的值等同于 FALSE，那么它会被认为不存在。如果变量不存在的话，empty()并不会产生警告。

isset() 函数用于检测变量是否已设置并且非 NULL。如果已经使用 unset() 释放了一个变量之后，再通过 isset() 判断将返回 FALSE。若使用 isset() 测试一个被设置成 NULL 的变量，将返回 FALSE。同时要注意的是 null 字符（""）并不等同于 PHP 的 NULL 常量。

对于 0、false、空字符串、空数组的判断上，empty() 可以识别为空，isset() 则不能识别。

1、对于未设置的变量的判断

// 对于未设置的变量的判断if (empty($a)) {    echo "empty：变量为空" . "<br>";} else {    echo "empty：变量不为空" . "<br>";}if (isset($a)) {     echo "isset：变量已设置且不为空" . "<br>";} else {    echo "isset：变量未设置或变量为空" . "<br>";}##结果empty：变量为空isset：变量未设置或变量为空

2、对于 "" (空字符串) 的判断

// 对于 "" (空字符串) 的判断$a = "";if (empty($a)) {    echo "empty：变量为空" . "<br>";} else {    echo "empty：变量不为空" . "<br>";}if (isset($a)) {    echo "isset：变量已设置且不为空" . "<br>";} else {    echo "isset：变量未设置或变量为空" . "<br>";}##结果empty：变量为空isset：变量已设置且不为空

3、对于 0 (作为整数的0) 的判断

// 对于 0 (作为整数的0) 的判断$a = 0;if (empty($a)) {    echo "empty：变量为空" . "<br>";} else {    echo "empty：变量不为空" . "<br>";}if (isset($a)) {    echo "isset：变量已设置且不为空" . "<br>";} else {    echo "isset：变量未设置或变量为空" . "<br>";}##结果empty：变量为空isset：变量已设置且不为空

4、对于 0.0 (作为浮点数的0) 的判断

// 对于 0.0 (作为浮点数的0) 的判断$a = 0.0;if (empty($a)) {    echo "empty：变量为空" . "<br>";} else {    echo "empty：变量不为空" . "<br>";}if (isset($a)) {    echo "isset：变量已设置且不为空" . "<br>";} else {    echo "isset：变量未设置或变量为空" . "<br>";}##结果empty：变量为空isset：变量已设置且不为空

5、对于 "0" (作为字符串的0) 的判断

// 对于 "0" (作为字符串的0) 的判断$a = "0";if (empty($a)) {    echo "empty：变量为空" . "<br>";} else {    echo "empty：变量不为空" . "<br>";}if (isset($a)) {    echo "isset：变量已设置且不为空" . "<br>";} else {    echo "isset：变量未设置或变量为空" . "<br>";}##结果empty：变量为空isset：变量已设置且不为空

6、对于 NULL 的判断

// 对于 NULL 的判断$a = null;if (empty($a)) {    echo "empty：变量为空" . "<br>";} else {    echo "empty：变量不为空" . "<br>";}if (isset($a)) {    echo "isset：变量已设置且不为空" . "<br>";} else {    echo "isset：变量未设置或变量为空" . "<br>";}##结果empty：变量为空isset：变量未设置或变量为空

7、对于 FALSE 的判断

// 对于 FALSE 的判断$a = false;if (empty($a)) {    echo "empty：变量为空" . "<br>";} else {    echo "empty：变量不为空" . "<br>";}if (isset($a)) {    echo "isset：变量已设置且不为空" . "<br>";} else {    echo "isset：变量未设置或变量为空" . "<br>";}##结果empty：变量为空isset：变量已设置且不为空

8、对于 array() (一个空数组) 的判断 

// 对于 array() (一个空数组) 的判断$a = array();if (empty($a)) {    echo "empty：变量为空" . "<br>";} else {    echo "empty：变量不为空" . "<br>";}if (isset($a)) {    echo "isset：变量已设置且不为空" . "<br>";} else {    echo "isset：变量未设置或变量为空" . "<br>";}##结果empty：变量为空isset：变量已设置且不为空

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