您得到NULL响应的原因是JSON主体无法解析。
您会得到以下错误："We could not parse the JSON body of your request. (HINT: This likely means you aren't using your HTTP library correctly. The OpenAI API expects a JSON payload, but what was sent was not valid JSON. If you have trouble figuring out how to fix this, please send an email to support@openai.com and include any relevant code you'd like help with.)".
改变这个...

$post_fields = [
    "model" => "gpt-3.5-turbo",
    "messages" => ["role" => "user","content" => $query],
    "max_tokens" => 12,
    "temperature" => 0
];

......到这个。

$post_fields = array(
    "model" => "gpt-3.5-turbo",
    "messages" => array(
        array(
            "role" => "user",
            "content" => $query
        )
    ),
    "max_tokens" => 12,
    "temperature" => 0
);

工作示例

如果您在CMD中运行php test.php，OpenAI API将返回以下完成：
字符串（40）"
英国的首都是伦敦。"

• • 测试. php**

<?php
    $ch = curl_init();

    $url = 'https://api.openai.com/v1/chat/completions';

    $api_key = 'sk-xxxxxxxxxxxxxxxxxxxx';

    $query = 'What is the capital city of England?';

    $post_fields = array(
        "model" => "gpt-3.5-turbo",
        "messages" => array(
            array(
                "role" => "user",
                "content" => $query
            )
        ),
        "max_tokens" => 12,
        "temperature" => 0
    );

    $header  = [
        'Content-Type: application/json',
        'Authorization: Bearer ' . $api_key
    ];

    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($post_fields));
    curl_setopt($ch, CURLOPT_HTTPHEADER, $header);

    $result = curl_exec($ch);
    if (curl_errno($ch)) {
        echo 'Error: ' . curl_error($ch);
    }
    curl_close($ch);

    $response = json_decode($result);
    var_dump($response->choices[0]->message->content);
?>