what is the difference between collection and array in php (propel)?

Question

Collections And On-Demand Hydration
The advantage of using a collection instead of an array is that Propel can hydrate model objects on demand. Using this feature, you'll never fall short of memory when retrieving a large number of results. Available through the setFormatter() method of Model Queries, on-demand hydration is very easy to trigger:

<?php
 $authors = AuthorQuery::create()
   ->limit(50000)
   ->setFormatter(ModelCriteria::FORMAT_ON_DEMAND)
   ->find();

foreach ($authors as $author) {
  echo $author->getFirstName();
}

1) What is the meaning of "Hydration" here?
2) what is the difference between collection and array?

Source : Propel @1.6

Answer 1

1.Hidration

A means to improve performance by "filling" your Class/Object with the row data just when you need it.

Instead of doing "SELECT * FROM SomeTable" from a very large table, Propel will initially fire off "SELECT ID FROM SomeTable", then inside the loop, then do "SELECT [COLUMS] FROM SomeTable WHERE ID=[CurrentID]", hence "On Demand"

2. Collection vs Array Array is just the normal array, whereas a PropelCollection is an Object of Objects, which has a lot of stuff available such as:

• Paging results
• Checking for odd or even, ->isOdd(), etc.
• Checking the number of items in it $object->count()
• Format converting ->toYAML(), ->toCSV(), ->toXML()

Each item in the collection is a PropelObject, so you can still fetch data with ->getColumn() inside your loop. Documentation