std::chrono is used in C to process time, including obtaining the current time, measuring execution time, operation time point and duration, and formatting analysis time. 1. Use std::chrono::system_clock::now() to obtain the current time, which can be converted into a readable string, but the system clock may not be monotonous; 2. Use std::chrono::steady_clock to measure the execution time to ensure monotony, and convert it into milliseconds, seconds and other units through duration_cast; 3. Time point (time_point) and duration (duration) can be interoperable, but attention should be paid to unit compatibility and clock epoch (epoch)

ToaccessenvironmentvariablesinPHP,usegetenv()orthe$_ENVsuperglobal.1.getenv('VAR_NAME')retrievesaspecificvariable.2.$_ENV['VAR_NAME']accessesvariablesifvariables_orderinphp.iniincludes"E".SetvariablesviaCLIwithVAR=valuephpscript.php,inApach

PHPhasthreecommentstyles://,#forsingle-lineand/.../formulti-line.Usecommentstoexplainwhycodeexists,notwhatitdoes.MarkTODO/FIXMEitemsanddisablecodetemporarilyduringdebugging.Avoidover-commentingsimplelogic.Writeconcise,grammaticallycorrectcommentsandu

CTE is a temporary result set in MySQL used to simplify complex queries. It can be referenced multiple times in the current query, improving code readability and maintenance. For example, when looking for the latest orders for each user in the orders table, you can first obtain the latest order date for each user through the CTE, and then associate it with the original table to obtain the complete record. Compared with subqueries, the CTE structure is clearer and the logic is easier to debug. Usage tips include explicit alias, concatenating multiple CTEs, and processing tree data with recursive CTEs. Mastering CTE can make SQL more elegant and efficient.

Reasons and solutions for the header function jump failure: 1. There is output before the header, and all pre-outputs need to be checked and removed or ob_start() buffer is used; 2. The failure to add exit causes subsequent code interference, and exit or die should be added immediately after the jump; 3. The path error should be used to ensure correctness by using absolute paths or dynamic splicing; 4. Server configuration or cache interference can be tried to clear the cache or replace the environment test.

The method of using preprocessing statements to obtain database query results in PHP varies from extension. 1. When using mysqli, you can obtain the associative array through get_result() and fetch_assoc(), which is suitable for modern environments; 2. You can also use bind_result() to bind variables, which is suitable for situations where there are few fields and fixed structures, and it is good compatibility but there are many fields when there are many fields; 3. When using PDO, you can obtain the associative array through fetch (PDO::FETCH_ASSOC), or use fetchAll() to obtain all data at once, so the interface is unified and the error handling is clearer; in addition, you need to pay attention to parameter type matching, execution of execute(), timely release of resources and enable error reports.

In PHP, you can use a variety of methods to determine whether a string starts with a specific string: 1. Use strncmp() to compare the first n characters. If 0 is returned, the beginning matches and is not case sensitive; 2. Use strpos() to check whether the substring position is 0, which is case sensitive. Stripos() can be used instead to achieve case insensitive; 3. You can encapsulate the startsWith() or str_starts_with() function to improve reusability; in addition, it is necessary to note that empty strings return true by default, encoding compatibility and performance differences, strncmp() is usually more efficient.

WhensettingupMySQLtables,choosingtherightdatatypesiscrucialforefficiencyandscalability.1)Understandthedataeachcolumnwillstore—numbers,text,dates,orflags—andchooseaccordingly.2)UseCHARforfixed-lengthdatalikecountrycodesandVARCHARforvariable-lengthdata