Asymmetric encryption example

keyholder.php

<?php

class KeyHolder
{
    private $key;

    public function __construct()
    {
        $this->key = new class () {
            private $resource;

            public function __construct()
            {
                $this->resource = openssl_pkey_new(array('private_key_bits' => 2048));
            }

            public function getPublic(): string
            {
                return openssl_pkey_get_details($this->resource)['key'];
            }

            public function getPrivate(): string
            {
                openssl_pkey_export($this->resource, $key);

                return $key;
            }
        };
    }

    public function getPublicKey(): string
    {
        return $this->key->getPublic();
    }

    public function encryptFor(string $publicKey, string $unencrypted): string
    {
        if (
            openssl_public_encrypt(
                $unencrypted,
                $encrypted,
                $publicKey
            )
        ) {
            return base64_encode($encrypted);
        }

        throw new Exception('Encryption failed');
    }

    public function decrypt(string $encrypted): string
    {
        if (
            openssl_private_decrypt(
                base64_decode($encrypted),
                $decrypted,
                $this->key->getPrivate()
            )
        ) {
            return $decrypted;
        }

        throw new Exception('Decryption failed');
    }
}

$service = new KeyHolder();
$partner = new KeyHolder();

$encrypted = $partner->encryptFor($service->getPublicKey(), 'This is a test of the emergency broadcast system.');

// this succeeds
var_dump($service->decrypt($encrypted));

// this fails, only the recipient can decrypt the message
var_dump($partner->decrypt($encrypted));