PHP in-depth understanding of the use of strtotime function_PHP tutorial

In the previous article , we mentioned that the strtotime function will produce some errors when using strtotime("-1 month") to calculate today's day of the previous month. situation,
therefore led to writing this article, which includes the following content:
•Some usages of strtotime function
•Basic principles of implementation of strtotime function
•strtotime(”-1 month ") Reasons for evaluation failure
Some usages of strtotime function
1. strtotime("JAN") and strtotime("January")
The two usages The effect is the same, both return today's day in the specified month. If there is no today in the specified month, it will be postponed to the next month. For example, to calculate February on 2011-03-31, the code is:

Copy the code The code is as follows:

echo date("Y-m-d H:i:s", strtotime("feb", strtotime("2011-03-31")));

The program will output: 2011-03-03 00:00:00. From the appearance point of view, this result may not necessarily be what we want, but it can also be regarded as a solution. What determines this solution? The strtotime function only calculates the month when calculating the month, which is equivalent to directly setting the month to the value of the specified month. For example, jan and January will have a corresponding internal value.
2. First keyword
first is an auxiliary keyword. It can be used in combination with keywords such as week, day, etc. that can specify confirmation values. For example, to find the first in 2011 Sunday:

Copy code The code is as follows:

echo date("Y-m-d H:i:s", strtotime(" second sunday", strtotime("2011-01-01"))), "
";

In the PHP source code, the combination of first, week and day is used are separated, that is, the first day corresponds to a processing operation. In the final C implementation, the value of day is specified as 1, that is, the d field in the time structure is specified as 1, as shown in the following code:

Copy code The code is as follows:

switch (time->relative.first_last_day_of) {
case 1: /* first */
time-> ;d = 1;
break;
case 2: /* last */
time->d = 0;
time->m++; Break;
}

3. Previous and next keywords
Similar to first, the previous keyword can be used in combination with week and day to indicate the day of the week or the day before the specified time one day. The code is as follows:

Copy code The code is as follows:

echo date("Y-m-d H:i:s", strtotime ("previous sunday", strtotime("2011-02-01"))), "
";

The program will output: 2011-01-30 00:00: 00
The program seeks the Sunday before 2011-02-01.
The next keyword is the opposite of previous, it indicates the next day of the week or the day after.

4. Last keyword
The last keyword can be used as either the previous one or the last one. For example, find the date of the previous Sunday:

Copy code The code is as follows:

echo date("Y-m-d H:i:s ", strtotime("last sunday", strtotime("2011-02-05"))), "
";

The program will output: 2011-01-30 00 :00:00
When the program is used as the last, its application scenario is the last day of the month where the specified date is located, which is equivalent to the result of date("t"). For example, the last day of February 2000:

Copy the code The code is as follows:

echo date("Y-m-d H:i :s", strtotime("last day", strtotime("2000-02-01"))), "
";

first, previous, last and this keys Words belong to the same group in the re file.

5. Back and front keywords
These two keywords are forward and backward operations on the hours of the day. The calling format is as follows:

Copy code The code is as follows:

echo date("Y-m-d H:i:s", strtotime("back of 24", strtotime("2011-02-01"))), "
";
echo date("Y-m-d H:i:s", strtotime("front of 24", strtotime("2011-02-01"))), "
";

•back means setting the time to 15 minutes of the hour after the specified hour value. If it is 24:00, it will be counted to 0:15 of the next day.
•front means setting the time to 45 minutes of the hour before the specified hour value. If it is 0 o'clock, it is calculated as 23:45 of the previous day.
The above code output: 2011-02-02 00:15:00 2011-02-01 23:45:00. The arrays followed by back of and front of must be greater than or equal to 0 and less than or equal to 24.
Basic principles for the implementation of strtotime function
The official document explains the strtotime function as follows: This function is expected to accept a string containing a US English date format and try This is parsed as a Unix timestamp (number of seconds since January 1 1970 00:00:00 GMT) relative to the time given by the now parameter, or the current system time if this parameter is not provided.
This is a standard PHP built-in function that has existed since PHP4. The strtotime function is loaded as an extension, and its full implementation is available in the ext/date directory. As a standard built-in function, its definition format is also standard, as follows:

Copy code The code is as follows:

PHP_FUNCTION( strtotime)
// Process input, whether there is a second parameter or not
// Call related functions to implement string parsing and result calculation
// Return result
}

In input processing, first identify the situation where both parameters exist and process it. If it is not in this state, process the situation where the second parameter does not exist. If neither parameter exists, an error will be reported. Return FALSE.
The first parameter of the strtotime function is a string. For this string, due to its complexity, PHP uses the same tool as its lexical parsing: re2c. In the /ext/date/lib directory, we can see its original re file from the parse_date.re file. When the user passes in a string in the form of a parameter, the string will be handed over to this program for processing, and different processing functions will be matched according to the different strings. For example, if strtotime("yesterday") is called, when analyzing the string, it will match the yesterday string. The corresponding function of this string is as follows:

Copy code The code is as follows:

'yesterday'
                                                      TIMELIB_UNHAVE_TIME();
s- >time->relative.d = -1; 🎜>

Copy code

The code is as follows:

typedef struct Scanner { 
         int           fd; 
         uchar        *lim, *str, *ptr, *cur, *tok, *pos; 
         unsigned int  line, len; 
         struct timelib_error_container *errors; 
         struct timelib_time *time; 
         const timelib_tzdb  *tzdb; 
     } Scanner; 
     typedef struct timelib_time { 
         timelib_sll      y, m, d;     /* Year, Month, Day */ 
         timelib_sll      h, i, s;     /* Hour, mInute, Second */ 
         double           f;           /* Fraction */ 
         int              z;           /* GMT offset in minutes */ 
         char            *tz_abbr;     /* Timezone abbreviation (display only) */ 
         timelib_tzinfo  *tz_info;     /* Timezone structure */ 
         signed int       dst;         /* Flag if we were parsing a DST zone */ 
         timelib_rel_time relative; 
         timelib_sll      sse;         /* Seconds since epoch */ 
         unsigned int   have_time, have_date, have_zone, have_relative, have_weeknr_day; 
         unsigned int   sse_uptodate; /* !0 if the sse member is up to date with the date/time members */ 
         unsigned int   tim_uptodate; /* !0 if the date/time members are up to date with the sse member */ 
         unsigned int   is_localtime; /*  1 if the current struct represents localtime, 0 if it is in GMT */ 
         unsigned int   zone_type;    /*  1 time offset,
                                       *  3 TimeZone identifier,
                                       *  2 TimeZone abbreviation */ 
     } timelib_time; 
     typedef struct timelib_rel_time { 
         timelib_sll y, m, d; /* Years, Months and Days */ 
         timelib_sll h, i, s; /* Hours, mInutes and Seconds */ 
         int weekday; /* Stores the day in 'next monday' */ 
         int weekday_behavior; /* 0: the current day should *not* be counted when advancing forwards; 1: the current day *should* be counted */ 
         int first_last_day_of; 
         int invert; /* Whether the difference should be inverted */ 
         timelib_sll days; /* Contains the number of *days*, instead of Y-M-D differences */ 
         timelib_special  special; 
         unsigned int   have_weekday_relative, have_special_relative; 
     } timelib_rel_time;

The reason why strtotime(”-1 month”) fails to evaluate
Although the method strtotime(”-1 month”) will fail to evaluate if the number of days in the next month is greater than the previous month, but from In essence, there is nothing wrong with this. There is nothing wrong with PHP's implementation. It's just that our needs dictate that we can't use this method, so we call it an evaluation failure.
Let’s look at its implementation process. Since there is no second parameter, the program uses the default current time. The first parameter is passed in the -1 month string. The regular pattern in the re file corresponding to this string is:

Copy code The code is as follows:

reltextunit = (('sec'|'second'|'min'|'minute'|'hour'|'day'|'fortnight'|'forthnight'|'month'|' year') 's'?) | 'weeks' | daytext;
relnumber = ([+-]*[ /t]*[0-9]+);
relative = relnumber space? (reltextunit | 'week' );

Finally relative will correspond to a series of operations. The program will recognize the -1 in front and the month string in the back. Month corresponds to an operation type: TIMELIB_MONTH. After this, the operation is performed based on the recognized number and operation type, as follows:

Copy the code The code is as follows:

case TIMELIB_MONTH: s->time->relative.m += amount * relunit->multiplier; break;

The above code directly records the relative value of the month minus one. But for situations like March 31st, where there is no 31st in February, the program will automatically calculate the date to the next month.

http://www.bkjia.com/PHPjc/327115.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/327115.htmlTechArticleIn the previous article on how to use PHP to calculate today's date in the previous month, we mentioned that the strtotime function uses strtotime( "-1 month") There will be some problems when finding today's day in the previous month, so...