Some things to clarify:
- MySQL timestamp field type doesn't store unix timestamps but rather a datetime-kind value.
- UNIX timestamp is a number of a regular int type.
- The timestamp you're talking about is not a regular unix timestamp but a timestamp with milliseconds.
therefore the correct answer would be
$timestamp = '1299762201428';
$date = date('Y-m-d H:i:s', substr($timestamp, 0, -3));