isset() 与 array_key_exists() 比较

1.对于数组值的判断不同，对于值为null或''或false,isset返回false，array_key_exists返回true；

2. 执行效率不同，isset是内建运算符，array_key_exists是php内置函数，isset要快一些，详细请参考，PHP函数实现原理及性能分析;

3.当用isset访问一个不存在索引数组值时，不会引起一个E_NOTICE的php错误消息；

4.array_key_exists 会调用get_defined_vars判断数组变量是否存在，isset不用；

测试代码：

<?php

function microtime_float(){
    list($usec, $sec) = explode(" ", microtime());
    return ((float)$usec + (float)$sec);
}

$test_arr= array('aa'=>'dd','bb'=>'','cc'=>null,'dd'=>false);

echo "isset aa is ";var_dump(isset($test_arr['aa']));echo "\n";
echo "isset bb is ";var_dump(isset($test_arr['bb']));echo "\n";
echo "isset cc is ";var_dump(isset($test_arr['cc']));echo "\n";
echo "isset dd is ";var_dump(isset($test_arr['cc']));echo "\n";
echo "isset none is ";var_dump(isset($test_arr['none']));echo "\n";

echo "key_exist aa is ";var_dump(array_key_exists('aa',$test_arr));echo "\n";
echo "key_exist bb is ";var_dump(array_key_exists('bb',$test_arr));echo "\n";
echo "key_exist cc is ";var_dump(array_key_exists('cc',$test_arr));echo "\n";
echo "key_exist dd is ";var_dump(array_key_exists('dd',$test_arr));echo "\n";
echo "key_exist none is ";var_dump(array_key_exists('none',$test_arr));echo "\n";
for($i=0;$i<10000;$i++){
    isset($test_arr['aa']);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "isset 10000 is $time\n";

for($i=0;$i<1000000;$i++){
    isset($test_arr['aa']);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "isset 1000000 is $time\n";

//++++++++++++++++++++++++++++++
for($i=0;$i<10000;$i++){
    array_key_exists('aa',$test_arr);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "array_key_exists 10000 is $time\n";

for($i=0;$i<1000000;$i++){
    array_key_exists('aa',$test_arr);
}
$time_end = microtime_float();
$time = $time_end - $time_start;
echo "array_key_exists 1000000 is $time\n";

运行结果如下：

可以看出  isset() 对应数组的value为 null false时，isset() 返回 false， 当对应数组value为   ''  或者 string 时，isset() 返回 true；

array_key_exists 只有数组不存在对应的key时，才返回 false，其余都返回true

另外，isset的执行效率会高于 array_key_exists