PHP8中的新函数：array_key_first()的多种应用场景

$arr = ['foo' => 'bar', 'hello' => 'world', 'key' => 'value'];
$first_key = array_key_first($arr);

echo $arr[$first_key]; // 输出 bar

$arr = [];
$first_key = array_key_first($arr);

if ($first_key === null) {
    echo '数组为空';
} else {
    echo '数组不为空';
}

$indexed_arr = [1, 2, 3];
$associative_arr = ['key1' => 'value1', 'key2' => 'value2'];

$is_indexed_arr = is_int(array_key_first($indexed_arr));
$is_associative_arr = is_string(array_key_first($associative_arr));

if ($is_indexed_arr) {
    echo '这是一个索引数组';
}

if ($is_associative_arr) {
    echo '这是一个关联数组';
}

$my_array = ['foo' => 'bar', 'bar' => 'foo', 'ice' => 'cream', 'cream' => 'ice'];
$new_array = [];

foreach ($my_array as $key => $value) {
    if (!isset($new_array[$value])) {
        $new_array[$value] = $key;
    }
}

print_r($new_array);

Array
(
    [bar] => foo
    [cream] => ice
)

$my_array = ['foo' => 'bar', 'bar' => 'foo', 'ice' => 'cream'];

for ($i = array_key_first($my_array); $i !== null; $i = next($my_array)) {
    echo $i . " => " . $my_array[$i] . "
";
}

foo => bar
bar => foo
ice => cream