How to Convert DateTime to String?

I have an array of DateTime objects and need to use them as strings in a function call. I have tried casting it like `$string_datetime = (string)$myDateTimeObject;` but that doesn't work. Searching has been unfruitful as well, as most people are asking how to convert a string to DateTime instead. My Code: ``` $start_date = new DateTime(); $end_date = new DateTime(); $end_date = $end_date->modify('+1 day'); // Add time range to request $request['time_range'] = Array ( 'start' => $start_date, 'end' => $end_date); ``` When calling a function which expects a string (it's an API call) I receive this error: Catchable fatal error: Object of class DateTime could not be converted to string What's the correct way of converting/extracting a string from a DateTime object?