sleep and wakeup magic methods

from an object php code tutorial

what is sleep and wakeup doing in this example

Please do not answer with php.net/sleep or php.net/wakeup because I am trying to learn from this example

<?php
class Employee{
  private $fname;
  private $date_of_birth;
  
  public function setFirstName($fname){
    $this->fname=$fname;
  }
  public function getFirstName(){
    return $this->fname;
  }
  public function setBirthDate($dob){
    $this->date_of_birth=$dob;
  }
  public function getBirthDate(){
    return $this->date_of_birth;
  }
  public function __sleep(){
    return array("fname"); //because of this, only name is serialized
  }
  public function __wakeup(){
    if($this->fname == "Marsha"){
      $this->date_of_birth="09-12-1983";
    }
  }
}
$e= new Employee();
$e->setFirstName("Marsha");
$e->setBirthDate("09-12-1983");
$data=serialize($e)."\n";
var_dump (unserialize($data));

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