array_column函数索引问题

$arr = [
    [0, 1],
    [2, 3],
    [4, 5],
    [6, 7],
];

$a1 = array_column($arr, '0');
$a2 = array_column($arr, 0);

print_r($a1);//Array ( )
print_r($a2);//Array ( [0] => 0 [1] => 2 [2] => 4 [3] => 6 )