An example demonstrating how to implement Serializable and JsonSerializable interfaces in PHP - Exclusive for the group: https://www.facebook.com/groups/HTML.CSSandJavaScript/

User.php

<?php //php 7.2.24

declare(strict_types = 1);

class User implements Serializable, JsonSerializable {
    private $id;        /** @var string user id */
    private $firstname; /** @var string user's firstname */
    private $lastname;  /** @var string user's lastname */
    private $email;     /** @var string user's email */

    /**
     * ***********************************************************************************
     * ***********************************************************************************
     *                   Just adding the basic functionality
     * ***********************************************************************************
     * ***********************************************************************************
     */

    /**
     * Initialize class fields
     * 
     * @param string $id set user id
     * @param string $firstname set user firstname
     * @param string $lastname set user lastname
     * @param string $email set user email
     * 
     * @return void
     * 
     */
    public function __construct(string $id, string $firstname, string $lastname, string $email) {
        $this->id = $id;
        $this->firstname = $firstname;
        $this->lastname = $lastname;
        $this->email = $email;
    }

    /**
     * ***********************************************************************************
     * ***********************************************************************************
     *                   Implementing the Serializable interface
     * ***********************************************************************************
     * ***********************************************************************************
     */

    /**
     * Returns a string representation of the object
     * 
     * @return string 
     * 
     */
    public function serialize() : string {
        //  NOTICE THAT WE'RE SEPARATING VALUES USING SEMICOLON
        $str = "";
        $str .= "$this->id;";
        $str .= "$this->firstname;";
        $str .= "$this->lastname;";
        $str .= "$this->email";
        return $str;
    }

    /**
     * Constructs an object from string representation
     * 
     * @param string $serialized the string representation of the object
     * 
     * @throws UnexpectedValueException if the given $serialized isn't a string
     * 
     * @return void
     */

    public function unserialize($serialized) : void {
        if (!is_string($serialized)) {
            $type = gettype($serialized);
            throw new UnexpectedValueException("User::unserialize(serialized = $serialized) failed because serialized parameter is expected to be a string but $type was given");
        
        }
        $info = explode(";", $serialized);  //  Remember how we separated values using semicolons !
        
        //  By Convention from serialization process:
        $this->id = $info[0];           //  First field is the user id
        $this->firstname = $info[1];    //  Second field is the user firstname
        $this->lastname = $info[2];     //  Third field is the user lastname
        $this->email = $info[3];        //  Fourth field is the user email
    }

    /**
     * ***********************************************************************************
     * ***********************************************************************************
     *                   Implementing the JsonSerializable interface
     * ***********************************************************************************
     * ***********************************************************************************
     */

     /**
      * Specify what data to serialize as JSON
      * 
      * 
      * @return array the fields to serialize
      *
      */

    public function jsonSerialize() {
        $info = [
            "id"        => $this->id,
            "firstname" => $this->firstname,
            "lastname"  => $this->lastname,
            "email"     => $this->email
        ];
        return $info;
    }
}

//  Let's try this ...

$user = new User("984ed1d7-a342-48f6-89de-fb0e2e4b394a", "John", "Doe", "john.doe@example.com");

//  Let's get the custom serialized string
$serialized = serialize($user);
echo "Serialization using serialize():\n";
echo $serialized . "\n";
echo "--------------------------------------------------\n";

//  Now let's unserialize it
echo "Deserialization using unserialize():\n";
$object = unserialize($serialized);
var_dump($object);
echo "--------------------------------------------------\n";

//  Now let's serialize the object using json_encode()
echo "Serialization using json_encode():\n";
$serialized = json_encode($user);
echo $serialized . "\n";
echo "--------------------------------------------------\n";

//  Now let's unserialize the object using json_decode()
echo "Deserialization using json_decode():\n";
$object = json_decode($serialized);
var_dump($object);
echo "--------------------------------------------------\n";

?>