如何在Symfony中处理AES密钥

I'm developing a web app using Symfony 3.4 with PHP 7.2 and a SQLite db.

I want to encrypt some fields of my entity using AES-256 using standard libraries - openssl or sodium - but I'm really not sure on how to properly do it so I'm asking for your guidance before I make many big awful mistakes:

1. what are the best practices for storing key and IV?
2. is it relevant where I put it as long as it is not accessible by the web browser? I'm thinking to put them in the config.yml but it feels wrong, very wrong
3. which library is more secure between openssl and sodium?

I'm using the following code where my secret is stored in the parameters.yml so it will no be visible if you push it to Git.

/**
 * SecurityHelper.
 *
 * @author Kengy Van Hijfte <development@kengy.be>
 */
class SecurityHelper
{
    /** @var  string $secret */
    private $secret;

    public function __construct($secret)
    {
        $this->secret = $secret;
    }

    /**
     * @param $text
     * @return string
     */
    public function encrypt($text)
    {
        if (null == $text)
            return null;

        // Generate an initialization vector
        $iv = openssl_random_pseudo_bytes(openssl_cipher_iv_length('aes-256-cbc'));
        // Encrypt the data using AES 256 encryption in CBC mode using our encryption key and initialization vector.
        $encrypted = openssl_encrypt($text, 'aes-256-cbc', $this->secret, 0, $iv);
        // The $iv is just as important as the key for decrypting, so save it with our encrypted data using a unique separator (::)
        return base64_encode($encrypted . '::' . $iv);
    }

    /**
     * @param $text
     * @return string
     */
    public function decrypt($text)
    {
        if (null == $text)
            return null;

        // To decrypt, split the encrypted data from our IV - our unique separator used was "::"
        list($encrypted_data, $iv) = explode('::', base64_decode($text), 2);
        return openssl_decrypt($encrypted_data, 'aes-256-cbc', $this->secret, 0, $iv);
    }
}