You want to know the day or week of the year, the day of the week, or the day of the month. For example, you want to print a special message every Monday, or on the first of every month.
Use the appropriate arguments to date( ) or strftime( ) , as shown in Example 3-17.
Finding days of the week, month, and year
The two functions date( ) and strftime( ) don't behave identically. Days of the year start with 0 for date( ), but with 1 for strftime( ). Table 3-4 contains all the day and week number format characters date( ) and strftime( ) understand.
To print out something only on Mondays, use the w formatting character with date( ) or the %w string with strftime( ), as in Example 3-18.
Checking for the day of the week
There are different ways to calculate week numbers and days in a week, so take care to choose the appropriate one. The ISO standard (ISO 8601), says that weeks begin on Mondays and that the days in the week are numbered 1 (Monday) through 7 (Sunday). Week 1 in a year is the first week in a year with a Thursday in that year. This means the first week in a year is the first week with a majority of its days in that year. These week numbers range from 01 to 53.
Other week number standards range from 00 to 53, with days in a year's week 53 potentially overlapping with days in the following year's week 00.
As long as you're consistent within your programs, you shouldn't run into any trouble, but be careful when interfacing with other PHP programs or your database. For example, MySQL's DAYOFWEEK( ) function treats Sunday as the first day of the week, but numbers the days 1 to 7, which is the ODBC standard. Its WEEKDAY( ) function, however, treats Monday as the first day of the week and numbers the days from 0 to 6. Its WEEK( ) function lets you choose whether weeks should start on Sunday or Monday, but it's incompatible with the ISO standard.
3.7.4. See Also
Documentation on date( ) at http://www.php.net/date and strftime( ) at http://www.php.net/strftime; MySQL's DAYOFWEEK( ), WEEKDAY( ), and WEEK( ) functions are documented at http://www.mysql.com/doc/D/a/Date_and_time_functions.html .