Почему скрипт возвращает ошибку unexpected token in json at position 0?

Отправляю форму на php-обработчик через ajax таким образом:

function send(elem) {

$('.err').remove();
$.ajax({
    url: '/index.php?route=common/home/send',
    type: 'post',
    data: $('.' + elem + ' input').serialize(),
    dataType: 'json',
    success: function (json) {
      
        if (json['telephone']) {
            $('#' + elem +' .btnsend').before('<div class="err">' + json['telephone'] + '</div>');
        }

        if (json['success']) {
            $('#' + elem + ' .modal-body').html('<p>'+json['success'] +'</p>');

            setTimeout(function () {
                $('#' + elem + ' .close').trigger('click');
            }, 2000);
        }
    },
    error: function (xhr, ajaxOptions, thrownError) {
        alert(thrownError + "\r\n" + xhr.statusText + "\r\n" + xhr.responseText);
    }
});
}

код на php:

public function send() {

    $json = array();

    if (isset($this->request->post['telephone']) && ((utf8_strlen($this->request->post['telephone']) > 17) || (utf8_strlen($this->request->post['telephone']) < 15))) {
        $json['telephone'] = 'Укажите корректный телефон!';
    }


    if (($this->request->server['REQUEST_METHOD'] == 'POST') && !$json) {
            
        $subject = $this->request->post['type'].' '.$this->config->get('config_name');

        $message = $this->request->post['type'] . "\n\n";

        if(isset($this->request->post['telephone']) && $this->request->post['telephone']) {
            $message .= 'Телефон: ' . $this->request->post['telephone'] . "\n\n";
        }

        if(isset($this->request->post['name']) && $this->request->post['name']) {
            $message .= 'Имя: ' . $this->request->post['name'] . "\n\n";
        }
    

        $message .= $this->config->get('config_name');

        $mail = new Mail();
        $mail->protocol = $this->config->get('config_mail_protocol');
        $mail->parameter = $this->config->get('config_mail_parameter');
        $mail->hostname = $this->config->get('config_smtp_host');
        $mail->username = $this->config->get('config_smtp_username');
        $mail->password = $this->config->get('config_smtp_password');
        $mail->port = $this->config->get('config_smtp_port');
        $mail->timeout = $this->config->get('config_smtp_timeout');             
        $mail->setTo($this->config->get('config_email'));
        $mail->setFrom($this->config->get('config_email'));
        $mail->setSender($this->config->get('config_name'));
        $mail->setSubject(html_entity_decode($subject, ENT_QUOTES, 'UTF-8'));
        $mail->setText(html_entity_decode($message, ENT_QUOTES, 'UTF-8'));
        $mail->send();

        $emails = explode(',', $this->config->get('config_mail_alert'));
        
        foreach ($emails as $email) {
            if ($email && preg_match('/^[^\@]+@.*\.[a-z]{2,6}$/i', $email)) {
                $mail->setTo(trim($email));
                $mail->send();
            }
        }   

        $json['success'] = 'Ваша заявка успешно отправлена!';

    }

    $this->response->addHeader('Content-Type: application/json');
    $this->response->setOutput(json_encode($json));

}

}

CMS использую opencart. Почему он выдает ошибку unexpected token in json at position 0 ? На php же делаю json_encode