How to create custom validation helpers in CakePHP?

@deron 

CakePHP provides various built-in validation rules to validate the data. However, sometimes we may need to create our custom validation rules as per the specific requirements of our application. In CakePHP, we can create custom validation helpers to validate the data.

Here are the steps to create custom validation helpers in CakePHP:

1. Create a new file in the src/Validation/Validation.php directory of your CakePHP application. Name this file after the validation rule you want to create, for example, CustomValidation.php.
2. Inside the CustomValidation.php file, define a static method that accepts two arguments: $value and $options. $value represents the data being validated, and $options represents any additional options that may be needed to validate the data.
3. In the method, perform the validation as per the requirements of your application. If the data is valid, return true, otherwise, return an error message as a string.
4. Finally, register your custom validation helper in your application's validation method. For example:

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<?php
// src/Validation/CustomValidation.php

namespace AppValidation;

use CakeValidationValidation;

class CustomValidation
{
    public static function myCustomValidation($value, $options)
    {
        if(/* validation passes */) {
            return true;
        }
        return 'Validation failed';
    }
}

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// src/Validation/AppValidation.php

namespace AppValidation;

use CakeValidationValidator;

class AppValidator extends Validator
{
    public function __construct()
    {
        parent::__construct();

        // Register custom validation rule
        Validation::add('myCustomValidation', 'AppValidationCustomValidation::myCustomValidation');
    }
}

Now you can use your custom validation helper myCustomValidation in your validation rules just like any other built-in validation rule. For example:

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// In your table class

public function validationDefault(Validator $validator)
{
    $validator
        ->add('field_name', [
            'my_custom_validation' => [
                'rule' => 'myCustomValidation',
                'message' => 'Custom validation failed'
            ]
        ]);

    return $validator;
}

That's it! You have successfully created a custom validation helper in CakePHP.