I really like the strtotime() function, but the user manual doesn\'t give a complete description of the supported date formats. strtotime(\'dd/mm/YYYY\')<
You can parse dates from a custom format (as of PHP 5.3) with DateTime::createFromFormat
$timestamp = DateTime::createFromFormat('!d/m/Y', '23/05/2010')->getTimestamp();
(Aside: The ! is used to reset non-specified values to the Unix timestamp, ie. the time will be midnight.)
If you do not want to (or cannot) use PHP 5.3, then a full list of available date/time formats which strtotime accepts is listed on the Date Formats manual page. That page more thoroughly describes the fact that m/d/Y is inferred over d/m/Y (but you can, as mentioned in the answers here, use d-m-Y, d.m.Y or d\tm\tY).
In the past, I've also resorted to the quicky str_replace mentioned in another answer, as well as self-parsing the date string into another format like
$subject = '23/05/2010';
$formatted = vsprintf('%3$04d/%2$02d/%1$02d', sscanf($subject,'%02d/%02d/%04d'));
$timestamp = strtotime($formatted);