json_encode() 是 PHP 中用于將數組或對象轉換為 JSON 格式的字符串的函數。以下是如何使用 json_encode() 實現數據格式轉換的示例：

1. 將關聯數組轉換為 JSON 字符串：

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

$json_string = json_encode($assoc_array);

echo $json_string;
?>

輸出：

{"name":"John","age":30,"city":"New York"}

2. 將多維數組轉換為 JSON 字符串：

<?php
$multi_dim_array = array(
    array(
        "name" => "John",
        "age" => 30,
        "city" => "New York"
    ),
    array(
        "name" => "Jane",
        "age" => 28,
        "city" => "San Francisco"
    )
);

$json_string = json_encode($multi_dim_array);

echo $json_string;
?>

輸出：

[
  {"name":"John","age":30,"city":"New York"},
  {"name":"Jane","age":28,"city":"San Francisco"}
]

3. 將對象轉換為 JSON 字符串：

<?php
class Person {
    public $name;
    public $age;
    public $city;

    public function __construct($name, $age, $city) {
        $this->name = $name;
        $this->age = $age;
        $this->city = $city;
    }
}

$person = new Person("John", 30, "New York");

$json_string = json_encode($person);

echo $json_string;
?>

輸出：

{"name":"John","age":30,"city":"New York"}

注意：json_encode() 函數在處理特殊字符（如非 ASCII 字符）時可能會返回 null 或拋出警告。為了避免這些問題，可以使用 JSON_UNESCAPED_UNICODE 選項來保留 Unicode 字符：

<?php
$json_string = json_encode($assoc_array, JSON_UNESCAPED_UNICODE);

echo $json_string;
?>

輸出：

{"name":"John","age":30,"city":"紐約"}