gistfile1.php

<?php

$teste = array(
  array("foo" => "a", "bar" => "b", "timestamp" => 123456),
  array("foo" => "a", "bar" => "b", "timestamp" => 987654),
  array("foo" => "a", "bar" => "b", "timestamp" => 654789),
  array("foo" => "a", "bar" => "b", "timestamp" => 456123)
);

function sortTimestamp($a, $b) {
    return $a['timestamp'] > $b['timestamp'] ? 1 : -1;
}

usort($teste, 'sortTimestamp');

print_r($teste);