<?php
$data = '[{"year":"2008","value":"4169.20"},
{"year":"2009","value":"4067.50"},
{"year":"2010","value":"4848.40"},
{"year":"2011","value":"5654.80"},
{"year":"2012","value":"6071.50"}]';

$a = json_decode($data);
$b = array();
foreach ($a as $key => $v) {
 $c['year'] = $v->year;
 $c['value'] = (float) $v->value;
 $b[] = $c;
}
echo '<pre>';
print_r(json_encode($b));
echo '</pre>';
?>

<pre>[{"year":"2008","value":4169.2},{"year":"2009","value":4067.5},{"year":"2010","value":4848.4},{"year":"2011","value":5654.8},{"year":"2012","value":6071.5}]</pre>