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。

<?php

if(0 == null){
    echo "0 == null".PHP_EOL;
}else{
    echo "0 != null".PHP_EOL;
}

if('' == null){
    echo "'' == null".PHP_EOL;
}else{
    echo "'' != null".PHP_EOL;
}

if('0' == null){
    echo "'0' == null".PHP_EOL;
}
else{
    echo "'0' != null".PHP_EOL;
}

if('0' == false){
    echo "'0' == false".PHP_EOL;
}
else{
    echo "'0' != false".PHP_EOL;
}

if(false == null){
    echo "false == null".PHP_EOL;
}else{
    echo "false != null".PHP_EOL;
}

if('0'){
    echo "'0' is true".PHP_EOL;
}else{
    echo "!'0' is true".PHP_EOL;
}

$a = [null,null,1];
if(isset($a[0])){
    echo '$a[0] isset true'.PHP_EOL;
}else{
    echo '$a[0] isset false'.PHP_EOL;
}
if(isset($a[2])){
    echo '$a[2] isset true'.PHP_EOL;
}else{
    echo '$a[2] isset false'.PHP_EOL;
}
if(isset($a[10])){
    echo '$a[10] isset true'.PHP_EOL;
}else{
    echo '$a[10] isset false'.PHP_EOL;
}

empty(), is_null(), isset()(区别)