A flip/flop unit test to give you a sample:

<?php

use PHPUnit\Framework\TestCase;

class SodiumTest extends TestCase
{
    public function testSodium()
    {
        // or 32 cryptographically secure bytes
        // store the key securely with other secrets in your app 
       $key = sodium_crypto_aead_xchacha20poly1305_ietf_keygen();

        // 8-bytes nonce should be stored along with the ciphertext (will be needed for decryption)
        // It is not sensitive, you may just prepend it before the ciphertext.
        $nonce = random_bytes(SODIUM_CRYPTO_AEAD_CHACHA20POLY1305_NPUBBYTES);

        $flip = 'Hello, world!';
        $ciphertext = sodium_crypto_aead_chacha20poly1305_encrypt($flip, $nonce, $nonce, $key);
        $flop = sodium_crypto_aead_chacha20poly1305_decrypt($ciphertext, $nonce, $nonce, $key);

        $this->assertEquals($flip, $flop);
    }
}

?>

Side note: the nonce is used twice in this test, but you can use a username, an identifier or whatever you like in `$additional_data`