PHP in-depth understanding of strtotime function_PHP tutorial

•Some uses of strtotime function
•Basic principles of implementation of strtotime function
•The reason why strtotime(”-1 month”) failed to evaluate
Some usages of strtotime function
1. strtotime("JAN") and strtotime("January")
The effect of these two usages is the same. They 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, if February is calculated on 2011-03-31, the code is:

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, and such as 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 Sunday in 2011:

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 combined use of first, week and day is separate, that is, 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. The following 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 day before the specified time. The code is as follows:

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 represents the next day of the week or the day after.
4. last keyword
The last keyword can be used as either the previous or the last one. For the date of last Sunday:

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"). Please find 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"))), "
";

The first, previous, last and this keywords 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, and their calling format is as follows:

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 to set 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 outputs: 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 of implementation of strtotime function
The official documentation describes the strtotime function as follows: This function is expected to accept a string containing a US English date format and try to parse it into a Unix timestamp (the number of seconds since January 1 1970 00:00:00 GMT), Its value is relative to the time given by the now parameter. If this parameter is not provided, the current system time is used.
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:

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 results
}

In the input processing, the situation where both parameters exist is first identified and processed. If it is not in this state, the situation where the second parameter does not exist is processed. If neither parameter exists, an error is reported and FALSE is returned.
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:

The code is as follows:
'yesterday'
{
DEBUG_OUTPUT("yesterday");
TIMELIB_INIT;
TIMELIB_HAVE_RELATIVE();
TIMELIB_UNHAVE_TIME();
s->time->relative.d = -1;
TIMELIB_DEINIT;
return TIMELIB_RELATIVE;
}

Here are several key structures:

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”) failed to evaluate
Although the strtotime("-1 month") method will fail to evaluate the number of days in the next month compared to the previous month, there is nothing wrong with this in essence. 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 expression in the re file corresponding to this string is:

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' );

In the end, 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, perform operations based on the identified numbers and operation types, as shown in the following 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.

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