<?php
 
$posts = array(
  array('date' => '7/30/10', 'title' => 'july post'),
  array('date' => '7/19/10', 'title' => 'another post in july'),
  array('date' => '7/22/10', 'title' => 'sup, this will be with another post')
);
 
$new_posts = array();
 
foreach( $posts as $post ) {
  $week = date('W', strtotime($post['date']));
 
  if( !isset($new_posts[$week]) ) {
    $new_posts[$week] = array();
  }
 
  $new_posts[$week][] = $post;
}
 
print_r($new_posts);

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