convert date string to mysql datetime field

First, convert the string into a timestamp:

$timestamp = strtotime($string);

Then do a

date("Y-m-d H:i:s", $timestamp);

If these strings are currently in the db, you can skip php by using mysql's STR_TO_DATE() function.

I assume the strings use a format like month/day/year where month and day are always 2 digits, and year is 4 digits.

UPDATE some_table
   SET new_column = STR_TO_DATE(old_column, '%m/%d/%Y')

You can support other date formats by using other format specifiers.

Use DateTime::createFromFormat like this :

$date = DateTime::createFromFormat('m/d/Y H:i:s', $input_string.' 00:00:00');
$mysql_date_string = $date->format('Y-m-d H:i:s');

You can adapt this to any input format, whereas strtotime() will assume you're using the US date format if you use /, even if you're not.

The added 00:00:00 is because createFromFormat will use the current date to fill missing data, ie : it will take the current hour:min:sec and not 00:00:00 if you don't precise it.

$time = strtotime($oldtime);

Then use date() to put it into the correct format.