Parse it and put it back together again and then reformat it.
Try this function for the first part:
PHP: date_parse_from_format - Manual
and read this for the rest
PHP: date_format - Manual

function ChangeFormat($InputDate){

$TheDate=explode("/",$InputDate);
$TheDay=$TheDate[0];
$TheMonth=$TheDate[1];
$TheYear=$TheDate[2];

if (strlen($TheMonth)<2) $TheMonth="0".$TheMonth;
if (strlen($TheDay)<2) $TheDay="0".$TheDay;

return "$TheYear-$TheMonth-$TheDay";
} //ChangeFormat Ends

$IniDate="18/4/2013";
echo "Inital date is: $IniDate and Ending date format is: ".ChangeFormat($IniDate);


I hope this helps