Implementing Case-Insensitive Unique Validation in Yii2

I'm trying to make the unique rule perform a case-insensitive check. If the database contains the string hello, then inputs like Hello or heLLo should be considered duplicates, and the validation should return an error.

Here is what I tried but not working.

    public function rules()
    {
        return
        [
            ['code', 'unique', 'filter', 'filter' => 'strtolower', 'message' => 'This code already exists.'],
        ];
    }

Here is another solution I found

public function rules()
{
    return
    [
        [
            'code', 'unique', 'targetClass' => self::class, 
            'filter' => function ($query) {
                $query->andWhere('LOWER(code) = LOWER(:code)', [':code' => $this->code]);
            },
            'message' => 'This code already exists (case-insensitive).'
        ]
    ];
}

Can anyone please guide me what I am doing wrong.

I am able to get it working. Posting my solution in case anyone face similar issue.

Here is how my model code looks like

    public function rules()
    {
        return
        [
            ['code','validateCaseInsensitive']
        ];
    }

    public function validateCaseInsensitive($attribute, $params)
    {
        $query = self::find()
            ->where('LOWER(code) = LOWER(:code)', [':code' => strtolower($this->$attribute)]);

        // Exclude current record on update
        if (!$this->isNewRecord) {
            $query->andWhere(['<>', 'id', $this->id]);
        }
        
        if ($query->exists()) {
            $this->addError($attribute, 'Code already exists.');
        }
    }