Parsing strings for date time information:
<?php
  $dt = new DateTime("2018-03-08 08:43:57");
?>

This function will not return the timestamp as an integer, but instead returns a DateTime object, which you can access (as string) through:

<?php
  $dt = new DateTime("2018-03-08 08:44:12");
  echo $dt->format( 'U' ), "\n";
?>
Output
1520498652

The DateTime class is what you can do the really cool things with.