Object of class DateTime could not be converted to string

Because $newDate is an object of type DateTime, not a string. The documentation is explicit:

Returns new DateTime object formatted according to the specified format.

If you want to convert from a string to DateTime back to string to change the format, call DateTime::format at the end to get a formatted string out of your DateTime.

$newDate = DateTime::createFromFormat("l dS F Y", $dateFromDB);
$newDate = $newDate->format('d/m/Y'); // for example

Try this:

$Date = $row['valdate']->format('d/m/Y'); // the result will 01/12/2015

NOTE: $row['valdate'] its a value date in the database

Use this: $newDate = $dateInDB->format('Y-m-d');