csb.php

<?php
//ARRAY IN:

$in=array(
  array('name' => 'john', 'occupation' => 'electrician'),
  array('name' => 'jack', 'occupation' => 'plumber'),
  array('name' => 'jake', 'occupation' => 'office rat'),
  array('name' => 'pete', 'occupation' => 'electrician')
);

//ARRAY OUT:
/*
array(
  'electrician' => array('john', 'pete'),
  'plumber' => array('jack'),
  'office rat' => array('jake')
);
*/

$out=array();

foreach($in as $val)
{
  if(!isset($out[$val['occupation']])&&!is_array($out[$val['occupation']]))
    $out[$val['occupation']]=array();
  
  $out[$val['occupation']][]=$val['name'];
}
?>