Yii framework is a powerful yet easy-to-use PHP framework that provides a wealth of functions and tools to help us develop web applications. Occasionally, user-entered data may be incorrect or invalid. To ensure that the data entered by the user is correct, the Yii framework provides a feature called form validation.

Form validation is a very useful function that can automatically verify whether the data entered by the user conforms to the rules we define. These rules can contain required fields, length limits, rule validation, and more. If the data entered by the user does not comply with these rules, the Yii framework will display an error message to let the user know that the data they entered is incorrect.

In the Yii framework, form validation is implemented by using model classes. A model class is a class used to handle data and business rules. It can save data, validate data, interact with data with the database, and more. We can create a model class according to our needs and define rules in this class to validate the form data.

The following is an example of a model class:

namespace appmodels;

use Yii;
use yiiaseModel;

class ContactForm extends Model
{
    public $name;
    public $email;
    public $subject;
    public $body;
    public $verifyCode;

    public function rules()
    {
        return [
            // name, email, subject and body are required
            [['name', 'email', 'subject', 'body'], 'required'],
            // email has to be a valid email address
            ['email', 'email'],
            // verifyCode needs to be entered correctly
            ['verifyCode', 'captcha'],
        ];
    }

    public function attributeLabels()
    {
        return [
            'name' => 'Name',
            'email' => 'Email',
            'subject' => 'Subject',
            'body' => 'Body',
            'verifyCode' => 'Verification Code',
        ];
    }
}

In this model class, we define five attributes: name, email, subject, body and verifyCode. We also define rules to validate the values ??of these properties. These rules specify validation rules for each attribute, such as required field validation, email address validation, and captcha validation.

The rules() method in this model class returns an array containing all the validation rules we defined. In this example, we use the validation rules provided by the Yii framework, such as required field validation ('required') and email address validation ('email').

In addition, we can also use custom validation rules, defined through the callback function in the rules() method. For example, we can use the following code to define a custom validation rule:

['name', 'myRule'],

where myRule is a custom validation rule that is defined in another method in the model class. For example:

public function myRule($attribute, $params)
{
    if ($this->$attribute == 'test') {
        $this->addError($attribute, 'The value cannot be "test".');
    }
}

In this custom validation rule, we check whether the value of the attribute name is "test" and display an error message if so.

In the model class, we also need to define attribute labels so that the labels of the form fields are displayed correctly on the page. This can be achieved using the attributeLabels() method. For example:

public function attributeLabels()
{
    return [
        'name' => 'Name',
        'email' => 'Email Address',
        'subject' => 'Subject',
        'body' => 'Content',
        'verifyCode' => 'Verification Code',
    ];
}

Using this model class, we can easily perform form validation in Yii framework. We just need to use the following code in the controller:

$model = new ContactForm();

if ($model->load(Yii::$app->request->post()) && $model->validate()) {
    // data is valid, do something
}

In this code, we first create a new ContactForm object. We then load the form data into the model by calling the load() method. Finally, we validate the form data by calling the validate() method. If the form data is valid, we can use the $model object to process the data, e.g. store them into a database.

In summary, form validation in the Yii framework is a very useful function, which can automatically verify whether the data entered by the user conforms to the rules we define. To implement form validation, we need to create a model class and define validation rules in this class. Using these validation rules, we can easily validate form data in the Yii framework and ensure that user input is correct.

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