what is the difference between collection and array in php (propel)?
<p><strong>Collections And On-Demand Hydration</strong> <br>
The advantage of using <em>a collection instead of an array</em> 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:</p>

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

foreach ($authors as $author) {
  echo $author-&gt;getFirstName();
}
</code></pre>

<p>1) <strong>What is the meaning of "Hydration" here?</strong> <br>
2) <strong>what is the difference between collection and array?</strong></p>

<p>Source : <a href="http://propelorm.org/Propel/documentation/03-basic-crud.html#collections-and-on-demand-hydration" rel="nofollow">Propel @1.6</a></p>