How to convert date to timestamp in PHP?

$timestamp = strtotime('22-09-2008');

$a = strptime('22-09-2008', '%d-%m-%Y');
$timestamp = mktime(0, 0, 0, $a['tm_month'], $a['tm_day'], $a['tm_year']);

list($day, $month, $year) = explode('-', '22-09-2008');
echo mktime(0, 0, 0, $month, $day, $year);

$dateTime = new DateTime('2011-05-17');
echo $dateTime->format('U');

// or

$date = date_create('2011-05-17');
echo date_format($date, 'U');

// or

$date = new DateTime('2011-05-17');
echo $date->getTimestamp();

// or

$date = date_create('2011-05-17');
echo date_timestamp_get($date);

$a = strptime('22-09-2008', '%d-%m-%Y');
$timestamp = mktime(0, 0, 0, $a['tm_mon']+1, $a['tm_mday'], $a['tm_year']+1900)

//returns an array containing day start and day end timestamps

$old_timezone=date_timezone_get();
date_default_timezone_set($timezone);
$date=strptime($date,$format);
$day_start=mktime(0,0,0,++$date['tm_mon'],++$date['tm_mday'],($date['tm_year']+1900));
$day_end=$day_start+(60*60*24);
date_default_timezone_set($old_timezone);
return array('day_start'=>$day_start, 'day_end'=>$day_end);

$date="30/07/2010 13:24"; //date example
list($day, $month, $year, $hour, $minute) = split('[/ :]', $date);
//the variables should be arranged acording to your date format and so the separators
$timestamp=mktime($hour, $minute,0, $month, $day, $year);
echo date("r", $timestamp);