What is Doctrine hydration?

I've read about hydration in doctrine's documentation but I still can't understand what it is.

Could someone please explain?

Hydration is a method used to return query results. For example:

1. HYDRATE_ARRAY - This will return you an array of records that are represented by another array:

   $q = Doctrine_Query::create()
      ->from('Post p')
      ->setHydrationMode(Doctrine::HYDRATE_ARRAY);
   
   $resultSet = $q->execute(); // $resultSet is an array
   
   foreach ($resultSet as $post) {
       // $post is an array
       echo $post['title'];
   }
   

2. HYDRATE_RECORD - This will return you an collection (Doctrine_Collection) of objects:

   $q = Doctrine_Query::create()
      ->from('Post p')
      ->setHydrationMode(Doctrine::HYDRATE_RECORD); // Unnecessary, HYDATE_RECORD is default method
   
   $resultSet = $q->execute(); // $resultSet is an Doctrine_Collection object
   
   foreach ($resultSet as $post) {
       // $post is an Post object
       echo $post->getTitle();
       echo $post['title']; // Each Doctrine's Model object implements ArrayAccess interface so this is possible
       echo $post->myCustomMethod();
   }
   

3. HYDRATE_SINGULAR_SCALAR - Will return the value of first column of query's result:

    $q = Doctrine_Query::create()
      ->select('p.created_at')
      ->from('Post p')
      ->where('p.id = ?', 321)
      ->setHydrationMode(Doctrine::HYDRATE_SINGULAR_SCALAR); 
   
   $createdAt = $q->execute(); // $createdAt has value of first column from first record from result set (eg.: 2008-04-06 21:22:35)
   

There is a few more methods , you can read about each in documentation.

$q->fetchOne(array(), Doctrine_Core::HYDRATE_ARRAY);

It will return just a simple array instead of a doctrine collection object.