convert unix timestamp php

I have a database that stores the time for me. I insert it from PHP using

date( 'Y-m-d H:i:s');

I then use this function to convert it to a unix timestamp in PHP

function convert_datetime($str) 
{
list($date, $time) = explode(' ', $str);
list($year, $month, $day) = explode('-', $date);
list($hour, $minute, $second) = explode(':', $time);

$timestamp = mktime($hour, $minute, $second, $month, $day, $year);

return $timestamp;
}

What I now need to do is convert this timestamp to the date and time in this format: yyyymmddhhmmss (without any hyphens, dashes, colons, etc).

Has anyone any ideas?

Copyright License：
Author:「109221793」,Reproduced under the CC 4.0 BY-SA copyright license with link to original source & disclaimer.
Link to：https://stackoverflow.com/questions/3707506/convert-unix-timestamp-php