SemicolonArrayType.php

<?php

namespace Acme\DemoBundle\DBAL\Type;

use Doctrine\DBAL\Types\Type;
use Doctrine\DBAL\Platforms\AbstractPlatform;

/**
 * Class SemicolonArrayType represents an array of values that will be persisted as a string of semicolon-separated strings.
 *
 * @see http://dev.mysql.com/doc/refman/5.7/en/string-functions.html#function_find-in-set
 * @package Acme\DemoBundle\Type
 */
class SemicolonArrayType extends Type
{
    /**
     * Name of this type
     */
    const TYPE = 'semicolon_array';
    const DELIMITER = ';';

    /**
     * @inheritDoc
     */
    public function getSqlDeclaration(array $fieldDeclaration, AbstractPlatform $platform)
    {
        return $platform->getClobTypeDeclarationSQL($fieldDeclaration);
    }

    /**
     * @inheritDoc
     */
    public function convertToPHPValue($value, AbstractPlatform $platform)
    {
        if ($value === null) {
            return null;
        }

        return explode(self::DELIMITER, $value);
    }

    /**
     * @inheritDoc
     */
    public function convertToDatabaseValue($value, AbstractPlatform $platform)
    {
        if (!is_array($value)) {
            return $value;
        }

        return implode(self::DELIMITER, $value);
    }

    /**
     * {@inheritdoc}
     */
    public function requiresSQLCommentHint(AbstractPlatform $platform)
    {
        return true;
    }

    /**
     * @inheritDoc
     */
    public function getName()
    {
        return self::TYPE;
    }
}

Symfony 2.4.x

doctrine:
    dbal:
        .....
        types:
            semicolon_array: Acme\DemoBundle\DBAL\Type\SemicolonArrayType
        mapping_types:
            semicolon_array: semicolon_array