PHP中如何获取关联数组变量的所有键名？

<?php
$arr = array("name" => "John", "age" => 30, "city" => "New York");

// 获取关联数组的所有键名
$keys = array_keys($arr);

// 输出键名
foreach($keys as $key) {
    echo $key . "\n";
}
?>

name
age
city

<?php ... ?>