二维数组变一维数组

<?php
　　$first_names = array_column($records, 'first_name');
　　var_dump($first_names);
?>
打印结果为：
$first_names = ['John','Sally','Jane','Peter'];

<?php
　　$first_names = array_column($records, 'first_name'，'id');
　　var_dump($first_names);
?>
打印结果为：
$first_names = [2135 =>'John',3245 => 'Sally',5342 => 'Jane',5623 => 'Peter'];