PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

480 Chapter 21 Managing the Date and Time
Because no specific built-in function is available for such calculations, an SQL query
to calculate the exact number of years is fairly complex. Here, we took a shortcut and
divided the age in days by 365.25 to give the age in years.This calculation can be one
year out if run on somebody’s birthday, depending on how many leap years there have
been in that person’s lifetime.
Using Microseconds
For some applications, measuring time in seconds is not precise enough to be useful. If
you want to measure very short periods, such as the time taken to run some or all of a
PHP script, you need to use the function microtime().
In PHP 5, you should pass true to microtime().When this optional parameter is
provided, microtime() will return the time as a floating point value that is ready for
whatever use you have in mind.The value is the same one returned by mktime(),
time(), or date() but has a fractional component.
The statement
echo number_format(microtime(true), 10, '.', '');
produces something like 1174091854.84.
On older versions, you cannot request the result as a float. It is provided as a string. A
call to microtime() without a parameter returns a string of this form "0.34380900
1174091816".The first number is the fractional part, and the second number is the number
of whole seconds elapsed since January 1, 1970.
Dealing with numbers rather than strings is more useful, so in PHP 5 it is easiest to
call microtime() with the parameter true.
Using the Calendar Functions
PHP has a set of functions that enable you to convert between different calendar systems.The
main calendars you will work with are the Gregorian, Julian, and Julian Day
Count.
Most Western countries currently use the Gregorian calendar.The Gregorian date
October 15, 1582, is equivalent to October 5, 1582, in the Julian calendar. Prior to that
date, the Julian calendar was commonly used. Different countries converted to the
Gregorian calendar at different times and some not until early in the twentieth century.
Although you may have heard of these two calendars, you might not have heard of
the Julian Day Count (JD). It is similar in many ways to a Unix timestamp. It is a count
of the number of days since a date around 4000 BC. In itself, it is not particularly useful,
but it is useful for converting between formats.To convert from one format to another,
you first convert to a Julian Day Count and then to the desired output calendar.
To use these functions under Unix, you first need to compile the calendar extension
into PHP with --enable-calendar.These functions are built into the standard
Windows install.