Usually the login in Contao is case-sensitive. But especially when using the email address of users as there username for the login this doesn't make sense. From a technical perspective it's understandable. But there are way to many users that type there email as they see fit. Especially if there is a name in the email address as for example: christian.kolb@liplex.de. Then there are multiple versions including Christian.Kolb@liplex.de.

The easiest way to enable a case-insensitive login in Contao is to change the collation from the default utf8_bin which is case-sensitive to utf8_general_ci which is case-insensitive.

In your tl_member.php in dca folder add the following line:

<?php

// Replace utf8_bin to utf8_general_ci to make username case-insensitive
$GLOBALS['TL_DCA']['tl_member']['fields']['username']['sql'] = 'varchar(64) COLLATE utf8_general_ci NULL';

That's it!

In addition to this I normalize the email addresses on registration. For this I build a custom hook which is used in the registration process. In it I transform the entered email address in the signup form to lowercase and store it as username and email address.

<?php

namespace AppBundle\Hooks;

use AppBundle\Entity\Member;
use AppBundle\Repository\MemberRepository;
use Contao\System;
use Doctrine\ORM\ORMException;

class MemberHooks
{
    /**
     * @var MemberRepository
     */
    private $memberRepository;

    /**
     * MemberDataContainer constructor.
     */
    public function __construct()
    {
        $doctrine = System::getContainer()->get('doctrine');

        $this->memberRepository = $doctrine->getRepository(Member::class);
    }

    /**
     * Set set username to be the same as the email address.
     *
     * @param int   $memberId
     * @param array $arrData
     * @param $element
     *
     * @throws ORMException
     */
    public function onCreateMember(int $memberId, array $arrData, $element): void
    {
        $member = $this->memberRepository->findOneById($memberId);

        if (null !== $member) {
            $emailLowercase = strtolower($member->getEmail());
            $member->setEmail($emailLowercase);
            $member->setUsername($emailLowercase);
            $this->memberRepository->store($member);
        }
    }
}

This hook of course has to be registered in the config.php in your config folder.

<?php

// Hooks
$GLOBALS['TL_HOOKS']['createNewUser'][] = [MemberHooks::class, 'onCreateMember'];