Web Database Applications With Php And Mysql (2nd Edition) [Electronic resources] نسخه متنی

3.4 Dates and Times

There are several PHP library
functions that work with dates and times. Most either generate a Unix
timestamp or format a Unix timestamp in a human-readable form.
Validation using dates, and working with the flexible PEAR Date
package, is discussed in Chapter 9. In this
section, we introduce timestamps and PHP library functions that work
with Dates and Times.

3.4.1 Generating a Timestamp

A Unix timestamp consists of the number of seconds since the
arbitrarily chosen time 1 January 1970 00:00:00 Greenwich Mean Time.
Most systems represent a timestamp using a signed 32-bit integer,
allowing a range of dates from December 13, 1901 through January 19,
2038. While timestamps are convenient to work with, care must be
taken when manipulating timestamps to avoid integer overflow errors.
While PHP automatically converts integers that overflow to floats,
these values aren't valid timestamps.

In the Microsoft Windows environment, the timestamp functions don't support negative timestamps and can be used only with dates between January 1, 1970 and January 19, 2038.

3.4.1.1 Current time

PHP provides several functions that
generate a Unix timestamp. The simplest:

integer time( )

returns the timestamp for the current date and time, as shown in this
fragment:

// prints the current timestamp: e.g., 1064699133
print time( );

3.4.1.2 Creating timestamps with mktime( ) and gmmktime( )

To create a timestamp for a past or future
date in the range December 13, 1901 through January 19, 2038, the
mktime( ) and gmmktime( )
functions are defined:

int mktime(int hour, int minute, int second, int month, int day, int year [, int is_dst])int gmmktime(int hour, int minute, int second, int month, int day, int year [, int is_dst])

Both create a timestamp from the parameters supplied; the parameters
supplied to gmmktime( ) represent a GMT date and
time, while the parameters supplied to mktime( )
represent the local time. This example creates a timestamp for 9:30
A.M. on June 18, 1998:

$aDate = mktime(9, 30, 0, 6, 18, 1998);

Both functions correctly handle parameter values that you might
consider out-of-range. For example, the following call passes 14 for
the month value, 29 for the day, and 2004 for the year, creating a
time stamp for 1 March 2005:

// Creates a time stamp for 1 March 2005
$aDate = mktime(0, 0, 0, 14, 29, 2004);

Setting the month to 14 and the year to 2004 overflows to February in
2005 and setting the day to 29 overflows to the first of March. This
characteristic allows scripts to add a quantum of time without range
checking. The following example shows how 30 days can be added to a
date and time:

$paymentPeriod = 30;  // Days
// generates a timestamp for 26 June 2002 by
// adding 30 days to 27 May 2002
$paymentDue = mktime(0, 0, 0, 5, 27 + $paymentPeriod, 2002);
// A different approach adds the appropriate number 
// of seconds to the timestamp for 27 May 2002
$paymentDue = mktime(0, 0, 0, 5, 27, 2002) 
+ ($paymentPeriod * 24 * 3600);

If the components of a date are outside the range of dates the
function is defined for, -1 is returned. Both functions allow the
supplied date to be interpreted as daylight savings time by setting
the flag is_dst to 1.

The order of the arguments to these functions is unusual and easily
confused. While the mktime( ) and
gmmktime( ) functions are similar to the Unix
mktime( ) function, the arguments
aren't in the same order.

3.4.1.3 String to timestamp

The strtotime(
) function generates a timestamp by parsing a
human-readable date and time (between December 13, 1901 and January
19, 2038) from the string time:

integer strtotime(string time)

The function interprets several standard representations of a date,
as shown here:

// Absolute dates and times
$var = strtotime("25 December 2002");
$var = strtotime("14/5/1955");
$var = strtotime("Fri, 7 Sep 2001 10:28:07 -1000");
// The current time: equivalent to time( )
$var = strtotime("now");
// Relative to now
print strtotime("+1 day"); // tomorrow
print strtotime("-2 weeks"); // two weeks ago
print strtotime("+2 hours 2 seconds"); // in two hours and two seconds

Care should be taken when using strtotime( )
with user-supplied dates. It's better to limit the
use of strtotime( ) to cases when the string to
be parsed is under the control of the script. For example,
it's used here to check a minimum age using a
relative date:

// date of birth: timestamp for 16 August, 1982
$dob = mktime(0, 0, 0, 8, 16, 1982);
// Now check that the individual is over 18
if ($dob < strtotime("-18 years"))
print "Legal to drive in the state of Victoria"; // prints

3.4.1.4 Subsecond times

A Unix timestamp
represents a date and time accurate to the second, but many
applications

require times to be represented to the
subsecond. PHP provides the function:

string microtime( )

The microtime( ) function returns a string that
contains both a Unix timestamp in seconds and a microsecond
component. The returned string begins with the microsecond component,
followed by the integer timestamp:

// prints the time now in the format "microsec sec"
// Example: 0.55512200 1064700291
print microtime( );

One common use of the function microtime( ) is
to generate an integer seed for a random-number generator:

// Generate a seed.
$seed = (float)microtime( ) * 1000000;
// prints (for example) 555206
print $seed;

Because the microsecond component appears at the start of the string
returned from microtime( ), the returned value
can be converted to a float with the (float) cast
operator. Multiplying the float result by 1,000,000 ensures that you
create a suitably varying integer.

The following fragment shows how you can use both the microsecond and
second components to create a floating point representation of the
time:

$parts = explode(" ", microtime( ));
$f = (float)$parts[0] + (int)$parts[1];
// prints (for example) 1064700291.56 
print $f;

3.4.2 Formatting a Date

While the Unix timestamp is useful in a
program, it isn't a convenient display format. The
date( ) and gmdate( )
functions return a human-readable formatted date and time:

string date(string format [, integer timestamp])string gmdate(string format [, integer timestamp])

The format of the returned string is determined by the
format argument. Passing in the optional
timestamp argument can format a predetermined
date. Otherwise, both functions format the current time. The format
string uses the formatting characters listed in Table 3-3 to display various components or
characteristics of the timestamp. To include characters without
having the function interpret them as formatting characters, escape
them with a preceding backslash character. The following examples
show various combinations:

// Set up a timestamp for 08:15am 24 Aug 1974
$var = mktime(8, 15, 25, 8, 24, 1974);
// "24/08/1974"
print date('d/m/Y', $var);
// "08/24/74"
print date('m/d/y', $var);
// prints "Born on Saturday 24th of August" 
print date('\B\o\r\n \o\n l jS \of F', $var);

PHP also provides the equivalent functions:

string strftime(string format [, integer timestamp])

string gmstrftime(string format [, integer timestamp])

The format string uses the same % sequence
formatting character sequences as the C library function
strftime( ). For example:

// Prints "24/08/1974 08:15:25"
print strftime("%d/%m/%Y %H:%M:%S", mktime(8, 15, 25, 8, 24, 1974));
// Prints "08/24/1974"
print strftime("%D", mktime(8, 15, 25, 8, 24, 1974));

The result of some % sequences used by strftime(
) and gmstrftime( ) depends on the
locale settings. The following sets the locale so the printed date
used the Estonian language and conventions:

setlocale (LC_TIME, 'es');
// prints "laupäev 24 august 1974"
print strftime ("%A %d %B %Y", mktime(8, 15, 25, 8, 24, 1974));

Formatting sequences supported by strftime( )
and gmstrftime( ) are shown in Table 3-4. Some of these % sequences are not supported
under Windows as noted.

3.4.3 Validating a Date

The function checkdate(
) returns true if a given
month, day, and
year form a valid Gregorian date:

boolean checkdate(integer month, integer day, integer year)

This function isn't based on a timestamp and so can
accept any dates in the years 1 to 32767. It automatically accounts
for leap years.

// Works for a wide range of dates
$valid = checkdate(1, 1, 1066); // true
$valid = checkdate(1, 1, 2929); // true
// Correctly identify bad dates
$valid = checkdate(13, 1, 1996); // false
$valid = checkdate(4, 31, 2001); // false
// Correctly handles leap years
$valid = checkdate(2, 29, 1996); // true
$valid = checkdate(2, 29, 2001); // false