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：
<?php function microtime_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 $time\n"; for($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"; //++++++++++++++++++++++++++++++ $time_start = microtime_float(); for($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 $time\n"; for($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";