This article aims to solve the problem that the $_POST variable is empty when the Yii2 framework receives POST requests of application/json type. The core is to understand how the web server handles different content types, and to guide developers to enable Yii2's automatic parsing of JSON request bodies by configuring yii\web\JsonParser. The article will provide detailed configuration examples, front-end JSON sending code, and back-end data acquisition methods to ensure that Yii2 applications can correctly process and utilize incoming data in JSON format.

Understand the root cause of POST data being empty

When sending a POST request using JavaScript's fetch API or other front-end technology, if the request header Content-Type is set to application/json, the back-end Yii2 application may not be able to obtain data by default through Yii::$app->request->post() or $_POST hyperglobal variable.

This is because when a web server (such as Apache, Nginx) processes HTTP requests, it only automatically parses request bodies of application/x-www-form-urlencoded or multipart/form-data and populates them into the $_POST variable. For requests of application/json type, the server treats them as the raw body and will not automatically resolve to a key-value pair. Therefore, when Yii2 obtains data from $_POST by default, it will naturally find that the data is empty.

Solution: Configure the JSON parser for Yii2

In order for Yii2 to correctly parse request bodies of application/json type, we need to configure yii\web\JsonParser in the application's request component. This parser reads the original request body and parses it into a PHP array or object, and Yii2 can access this data through standard methods such as Yii::$app->request->post() or Yii::$app->request->getBodyParams().

1. Configure web.php

In the configuration file of the Yii2 application (usually config/web.php), find the components section, modify the configuration of the request component, and add the parsers attribute:

 // config/web.php
'components' => [
    'request' => [
        // !!! insert a secret key in the following (if it is empty) - this is required by cookie validation
        'cookieValidationKey' => 'your-secret-key', // Please replace with your key 'parsers' => [
            'application/json' => 'yii\web\JsonParser',
            // If your front end may also send other JSON types, such as 'text/json', you can also add // 'text/json' => 'yii\web\JsonParser',
        ],
        // ... other request component configuration],
    // ...Other components]

Through the above configuration, when Yii2 receives a request with Content-Type application/json, it will automatically use yii\web\JsonParser to parse the request body.

2. Example of front-end sending JSON data

The following is an example of sending JSON data using the JavaScript fetch API. Please note that the Yii2 CSRF token acquisition and settings are included to ensure the security of the request.

 // Get the CSRF token and parameter name const csrfToken = document.querySelector("meta[name='csrf-token']").content;
const csrfParam = document.querySelector("meta[name='csrf-param']").content;

// Assume e.id and this.state.ChangeName are the data to be sent const dataToSend = {
    'id': e.id,
    'name': this.state.ChangeName
};

fetch("http://site.se/react/save-babysitter", {
    method: "POST",
    headers: {
        "Content-Type": "application/json", // explicitly specify the content type to JSON
        "Accept": "application/json",
        [csrfParam]: csrfToken, // Use the CSRF parameter name as the key, and the CSRF token as the value // "X-CSRF-Token": csrfToken // Another common CSRF token delivery method, Yii2 also supports },
    body: JSON.stringify(dataToSend) // Convert JavaScript objects to JSON string})
.then(response => {
    if (!response.ok) {
        // Handle HTTP errors, such as 404, 500
        throw new Error(`HTTP error! status: ${response.status}`);
    }
    return response.json(); // parse JSON response})
.then((data) => {
    console.log("Success:", data);
})
.catch((error) => {
    console.error("Error during fetch operation:", error);
});

Notice:

• Setting Content-Type: application/json in headers is the key.
• The body must be a string after JSON.stringify().
• The way CSRF tokens can be delivered may vary by Yii2 version or configuration, and the above example shows two common ways.

3. Get data from the backend controller

After configuring JsonParser, in the Yii2 controller, you can use the Yii::$app->request->post() method to get parsed JSON data like you get normal POST data. If you need to get all POST parameters, don't specify the key name.

 // controllers/ReactController.php (assuming your controller is called ReactController)

namespace app\controllers;

use Yii;
use yii\web\Controller;
use yii\filters\VerbFilter; // If you need to restrict the request method class ReactController extends Controller
{
    // If you need to disable CSRF verification (disabled in production environments is not recommended unless you understand the risks and have other security measures)
    public $enableCsrfValidation = false;

    public function behaviors()
    {
        Return [
            'verbs' => [
                'class' => VerbFilter::class,
                'actions' => [
                    'save-babysitter' => ['POST'], // Make sure only POST requests are allowed],
            ],
        ];
    }

    public function actionSaveBabysitter()
    {
        $request = Yii::$app->request;

        // Get the entire parsed JSON data (as an associative array)
        $data = $request->post();

        // Get a specific field, such as 'name'
        $name = $request->post('name');
        $id = $request->post('id');

        // Verify data (important steps!)
        if (empty($name) || empty($id)) {
            Yii::$app->response->statusCode = 400; // Bad Request
            return $this->asJson(['success' => false, 'message' => 'Missing necessary parameters']);
        }

        // Suppose the data is processed and saved to the database here // ...

        // Return JSON response return $this->asJson([
            'success' => true,
            'message' => 'Data received successfully',
            'received_data' => [
                'id' => $id,
                'name' => $name
            ]
        ]);
    }
}

Notes:

• CSRF Verification: By default, Yii2 performs CSRF verification on all POST requests. If you send JSON data through the fetch API, you need to make sure that the CSRF token is correctly passed in the request header or in the request body. In the above front-end code, we pass the CSRF token in the request header via [csrfParam]: csrfToken. If your API is stateless or session-dependent, and you understand the risks, you can disable CSRF verification at the controller or module level (such as $enableCsrfValidation = false; in the example), but this is not usually recommended.
• Data acquisition: After configuring JsonParser, Yii::$app->request->post() will automatically return the parsed JSON data. You can also use Yii::$app->request->getBodyParams() to get all request body parameters, or Yii::$app->request->getRawBody() to get the unparsed original request body string.
• Data Verification and Security: Even if the data is successfully received, it is necessary to strictly verify, filter and purify data on the backend to prevent security vulnerabilities such as SQL injection and XSS. Do not use the received data directly for database operations.
• Error handling: There should be a robust error handling mechanism on both the front-end and the back-end so that it can provide timely feedback and debug if the request fails or the data does not meet expectations.

Summarize

By configuring yii\web\JsonParser in the request component of the Yii2 application, we can easily solve the problem of application/json type POST request data being empty. This allows Yii2 to seamlessly interact with modern front-end frameworks (such as React, Vue, Angular) to build powerful front-end and back-end separation applications. Remember that data verification and security practices are always critical when processing any user input.

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