array_key_exists()函数判断某个数组中是否存在指定的key，如果key存在，则返回true，否则返回flase

array_key_exists(key,array);

key：必需。规定键名
array：必需。规定输入的数组

<?php$a = array('a'=>'Dog','b'=>'Cat');if(array_key_exists('a',$a)){ echo 'Key exists!';} else{ echo 'Key does not exist!';}?>

输出：Key exists!

array_key_exists为什么比in_array快？

array_key_exists 和 in_array 查询的东西都不一样吧
array_key_exists 判断是否有键值
array_key_exists(a,arr)->if(isset(arr[a]))就是true

而in_array 需要去遍历值 遍历到了才跳出循环

追问：
是不是数组的索引有单独的存储单元，而且优化过，array_key_exists的时间复杂度是o(1), 而in_array是o(n) ??

追答：
重复杂度来说是这样

array_key_exists 是判断某个键有没有值

in_array 要遍历一次 获取是否相同 不知道建的情况下必须遍历

PHP中isset与array_key_exists的区别

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。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：

<?phpfunctionmicrotime_float(){list($usec, $sec) = explode(" ", microtime());return ((float)$usec + (float)$sec);}$test_arr['aa']='dd';$test_arr['bb']='';$test_arr['cc']=NULL;$test_arr['dd']=false;$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";$time_start = microtime_float();for($i=0;$i<100;$i++){isset($test_arr['aa']);}$time_end = microtime_float();$time = $time_end - $time_start;echo "isset 100 is $timen";for($i=0;$i<10000;$i++){isset($test_arr['aa']);}$time_end = microtime_float();$time = $time_end - $time_start;echo "isset 10000 is $timen";for($i=0;$i<1000000;$i++){isset($test_arr['aa']);}$time_end = microtime_float();$time = $time_end - $time_start;echo "isset 1000000 is $timen";//++++++++++++++++++++++++++++++$time_start = microtime_float();for($i=0;$i<100;$i++){array_key_exists('aa',$test_arr);}$time_end = microtime_float();$time = $time_end - $time_start;echo "array_key_exists 100 is $timen";for($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 $timen";for($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 $timen";