- preg_split: documentation ( source)
- strtotime: documentation ( source)
<?php
$fecha = "20/04/2015 18:11:54";
list($d, $M, $y, $h, $m, $s) = preg_split("/[\s,\/:]+/",$fecha);
echo "\n->";
echo strtotime("$d-$M-$y $h:$m:$s") - 1429571959;
echo "\n->";
echo strtotime("2015-20-04 18:11:54") - 1429571959;
echo "\n->";
echo strtotime("20/04/2015 18:11:54") . 'No funciona';