Here are special meanings of some symbols which you can use in format string (first parameter)

* d - day of the month, numeric, 2 digits (with leading zeros)
* D - day of the week, textual, 3 letters; i.e. "Fri"
* F - month, textual, long; i.e. "January"
* h - hour, numeric, 12 hour format
* H - hour, numeric, 24 hour format
* i - minutes, numeric
* j - day of the month, numeric, without leading zeros
* l (lowercase 'L') - day of the week, textual, long; i.e. "Friday"
* m - month, numeric
* M - month, textual, 3 letters; i.e. "Jan"
* s - seconds, numeric
* S - English ordinal suffix, textual, 2 characters; i.e. "th", "nd"
* Y - year, numeric, 4 digits
* w - day of the week, numeric, 0 represents Sunday
* y - year, numeric, 2 digits
* z - day of the year, numeric; i.e. "299"
You can include any other symbols in format string and they should be outputted as they are. Only above symbols are replaced with their time and date values.
Examples:
<?
print(date( "l dS of F Y h:i:s" ));
?>
Output:
????Friday 04th of January 1994 3:52:31

<?
print(date("Current time is: H (hours), i (minutes), s (seconds)"));
?>
output:
????Current time is: 12 (hours), 34 (minutes), 13 (seconds)

mktime - get timestamp for date and time value
Syntax: int mktime (int hour, int minute, int second, int month, int day, int year);

Timestamp has been created to represent date / time value as a single integer number. So any logical date and time can be converted and used as special integer called timestamp. That's how you are able to compare two time moments as just two integer numbers. This timestamp contains the number of seconds between the Unix Epoch (January 1 1970) and the time specified. For example, January first of 1997th year will be less number than January second of that year.
In databases the date type fields usually stored as timestamp integers. So to create query to get all records in specified time period you can use mktime() function to get the boundary timestamp integers. If you combine both date and mktime function, you will be able to perform any time operation that you can imagine (and you don't need time machine or something :) )

Example:
Let's print date that happened 7 days ago:
<?
$m=date('m');
$d=date('d');
$Y=date('Y');
print(date('Y-m-d', mktime(0,0,0, $m, $d-7, $Y)));
?>
Note that PHP mktime()'s arguments sequence is different from standart UNIX mktime function. Most right arguments can be omitted and they will be set to current date time values. ?ktime also would make all needed corrections if after ariphmetic calculations you would get incorrect date, for example 32nd of Febrary :).
Example. All lines print "Jan-01-1998".
<?
print(date("M-d-Y", mktime(0,0,0,12,32,1997)));
print(date("M-d-Y", mktime(0,0,0,13,1,1997)));
print(date("M-d-Y", mktime(0,0,0,1,1,1998)));
?>