Encrypt/decrypt with libsodium

libsodium.php

<?php

function encrypt (string $message, string $key) : array 
{
    $nonce = random_bytes (SODIUM_CRYPTO_SECRETBOX_NONCEBYTES);

    $ciphertext = sodium_crypto_secretbox ($message, $nonce, base64_decode ($key));

    return [
        'nonce'         => base64_encode ($nonce),
        'ciphertext'    => base64_encode ($ciphertext)
    ];
}

function decrypt (string $ciphertext, string $nonce, string $key) : string 
{
    return sodium_crypto_secretbox_open ($ciphertext, base64_decode ($nonce), base64_decode ($key));
}

$message = "this is a string to be encrypted with libsodium.";
$key = base64_encode (sodium_crypto_secretbox_keygen());

var_dump ($key);
$payload = encrypt ($message, $key);
var_dump ($payload);
var_dump (decrypt ($payload['ciphertext'], $payload['nonce'], $key));