$i = serialize('20110510134021'); //i:20110510134021;

$i = unserialize('i:20110510134021;');

Notice: unserialize(): Error at offset 0 of 16 bytes

$i = 20110510134021;
echo $i;

$i = unserialize('d:20110510134021;');

$int = 20110510134021;
var_dump(serialize($int));

$serialized = 'i:20110510134021';

if (preg_match('/i:([d]+)/', $serialized, $match))
{
    $unserialized = $match[1];
}