// Assume you have a date string which is in d-m-Y format.
$date_string = '18-04-2013';

// Create the DateTime object
$date = date_create_from_format('d-m-Y', $date_string);

// Print the date in Y-m-d format.
print date('Y-m-d', $date->getTimestamp());

Done =)

Reference:

• StackOverflow – Convert this string to datetime
• PHP Manual – DateTime::createFromFormat