Simple PHP encryption

encryption.php

class Encryption
{
    const CIPHER =  'AES-128-CBC';

    /* Cryptographic key of length 16, 24 or 32. NOT a password! */
    private $key;
    private $iv_size;
    public function __construct($key) {
        $this->key = $key;
        $this->iv_size = openssl_cipher_iv_length($this::CIPHER);
    }

    public function encrypt($plaintext) {
        $iv = openssl_random_pseudo_bytes($this->iv_size);
        $ciphertext = openssl_encrypt($plaintext, $this::CIPHER, $this->key, OPENSSL_RAW_DATA, $iv);
        return base64_encode($iv.$ciphertext);
    }

    public function decrypt($ciphertext) {
        $ciphertext = base64_decode($ciphertext);
        if (strlen($ciphertext) < $this->iv_size) {
            throw new Exception('Missing initialization vector');
        }

        $iv = substr($ciphertext, 0, $this->iv_size);
        $ciphertext = substr($ciphertext, $this->iv_size);
        $plaintext = openssl_decrypt($ciphertext, $this::CIPHER, $this->key, OPENSSL_RAW_DATA, $iv);
        return rtrim($plaintext, "\0");
    }
}