php检测数组是否存在指定下标的两种方法

<?php
header('content-type:text/html;charset=utf-8');   
$arr=array("a"=>"Dog","b"=>"Cat");
echo "原数组：";
var_dump($arr);

$keys=array_keys($arr);
echo "键名数组：";
var_dump($keys);
var_dump(array_search("a",$keys));
var_dump(array_search("b",$keys));
var_dump(array_search("c",$keys));
?>

<?php
header('content-type:text/html;charset=utf-8');   
$arr=array("a"=>"Dog","b"=>"Cat");
var_dump($arr);
if (array_key_exists("a",$arr)){
    echo "指定下标'a'存在！";
}else
{
    echo "指定下标'a'不存在";
}
?>