strtotime()
doesn't understand that format, and hence returns FALSE
. The list of formats recognized by the parser are listed here.
Use DateTime::createFromFormat()
to parse this format:
$dateObj = DateTime::createFromFormat('d/m/Y g a', $chktodate);
echo $dateObj->format('Y-m-d H:i:s'); // 2014-02-15 09:00:00
For a list of all the available formatting options, see the documentation.