Explain the difference between `array_map`, `array_filter`, and `array_reduce`.

Singleton pattern ensures that a class has only one instance and provides a global access point for scenarios where a single object coordinates the operation of the system, such as database connections or configuration management. 2. Its basic structure includes: private static attribute storage instances, private constructors prevent external creation, private cloning methods prevent copying, and public static methods (such as getInstance()) for obtaining instances. 3. Get a unique instance in PHP by calling getInstance() method, and returns the same object reference no matter how many times it is called. 4. Under the standard PHP request model, thread safety is not necessary to be considered, but synchronization issues need to be paid attention to in long run or multi-threaded environments, and PHP itself does not support native lock mechanism. 5. Although singletons are useful,

Answer: PHP's empty merge operator (??) is used to check whether a variable or array key exists and is not null. If it is true, it returns its value, otherwise it returns the default value. It avoids the use of lengthy isset() checks, is suitable for handling undefined variables and array keys, such as $username=$userInput??'guest', and supports chain calls, such as $theme=$userTheme??$defaultTheme??'dark', which is especially suitable for form, configuration, and user input processing, but only excludes null values, empty strings, 0 or false are considered valid values ​​to return.

Use $_GET to get URL parameters, such as ?name=John&age=25; check existence through isset or empty merge operators, and filter and verify data with filter_input to ensure security.

Answer: Use file_get_contents and cURL to download URL files, the former is simple but restricted, while the latter is more flexible and supports streaming. Examples include directly reading and writing files, cURL initialization setting options and saving, adding error handling and HTTP status checking. Large files are recommended to stream download in blocks to save memory, ensuring that the directory is writable and handle exceptions properly.

TodisableaPHPfunction,usedisable_functionsinphp.iniforbuilt-infunctionslikeexecorsystem,whichblocksthemgloballyforsecurity;foruser-definedfunctions,preventexecutionbywrappingtheminconditions,renaming,commentingout,orcontrollingfileinclusionviaautoloa

Use the implements keyword to implement the interface, and the class must provide specific implementations of all methods in the interface. 2. Define the interface to declare the method using the interface keyword. 3. Class implements interface and overrides methods. 4. Create an object and call the method to output the result. 5. A class can implement multiple interfaces to ensure code specification and maintainability.

TopreventXSSinPHP,sanitizeuserinputandescapeoutputbasedoncontextusinghtmlspecialchars()forHTML,json_encode()forJavaScript,andvalidatestrictlywithfilter_var()forexpecteddatatypes,whileavoidingdeprecatedfunctionsandusingContent-Security-Policyheadersfo

The GET method attaches data to the URL, which is suitable for non-sensitive information; the POST method sends data through the request body, which is more secure and suitable for sensitive information.