Comments should explain "why" rather than "what was done", such as explaining business reasons rather than repeating code operations; 2. Add overview comments before complex logic, briefly explaining the process steps to help establish an overall impression; 3. Comments the "strange" code to explain the intention of unconventional writing, and avoid misunderstandings as bugs; 4. Comments are recommended to be concise, use // in single lines, use // in functions/classes/*.../ in order to maintain a unified style; 5. Avoid issues such as out of synchronization with the comments, too long comments or not deletion of the code, and ensure that the comments truly improve the readability and maintenance of the code.

When using if/else control structure for conditional judgment in PHP, the following points should be followed: 1. Use if/else when different code blocks need to be executed according to the conditions; 2. Execute if branches if the condition is true, enter else or elseif if they are false; 3. When multi-conditional judgment, elseif should be arranged in logical order, and the range should be placed in front of the front; 4. Avoid too deep nesting, it is recommended to consider switch or reconstruction above three layers; 5. Always use curly braces {} to improve readability; 6. Pay attention to Boolean conversion issues to prevent type misjudgment; 7. Use ternary operators to simplify the code in simple conditions; 8. Merge and repeat judgments to reduce redundancy; 9. Test boundary values to ensure the complete logic. Mastering these techniques can help improve code quality and stability.

PHP string processing requires mastering core functions and scenarios. 1. Use dot numbers or .= for splicing, and recommend arrays for splicing large amounts of splicing; 2. Use strpos() to search, replace str_replace(), pay attention to case sensitivity and regular usage conditions; 3. Use substr() to intercept, and use sprintf() to format; 4. Use htmlspecialchars() to output HTML, and use parameterized query to database operations. Familiar with these function behaviors can deal with most development scenarios.

The "undefinedindex" error appears because you try to access a key that does not exist in the array. To solve this problem, first, you need to confirm whether the array key exists. You can use isset() or array_key_exists() function to check; second, make sure the form data is submitted correctly, including verifying the existence of the request method and field; third, pay attention to the case sensitivity of the key names to avoid spelling errors; finally, when using hyperglobal arrays such as $_SESSION and $_COOKIE, you should also first check whether the key exists to avoid errors.

There are two ways to correctly use PHP annotation: // or # for single-line comments, and /.../ for multi-line comments. PHP syntax requires attention to the fact that each statement ends with a semicolon, add $ before the variable name, and case sensitivity, use dots (.) for string splicing, and maintain good indentation to improve readability. The PHP tag specification is for use to avoid unnecessary gaps. Mastering these basic but key details can help improve code quality and collaboration efficiency.

The key to setting up PHP is to clarify the installation method, configure php.ini, connect to the web server and enable necessary extensions. 1. Install PHP: Use apt for Linux, Homebrew for Mac, and XAMPP recommended for Windows; 2. Configure php.ini: Adjust error reports, upload restrictions, etc. and restart the server; 3. Use web server: Apache uses mod_php, Nginx uses PHP-FPM; 4. Install commonly used extensions: such as mysqli, json, mbstring, etc. to support full functions.

The key to writing PHP comments is to explain "why" rather than "what to do", unify the team's annotation style, avoid duplicate code comments, and use TODO and FIXME tags reasonably. 1. Comments should focus on explaining the logical reasons behind the code, such as performance optimization, algorithm selection, etc.; 2. The team needs to unify the annotation specifications, such as //, single-line comments, function classes use docblock format, and include @author, @since and other tags; 3. Avoid meaningless annotations that only retell the content of the code, and should supplement the business meaning; 4. Use TODO and FIXME to mark to do things, and can cooperate with tool tracking to ensure that the annotations and code are updated synchronously and improve project maintenance.

PHP has five most commonly used hyperglobal variables, namely $\_GET, $\_POST, $\_SERVER, $\_SESSION, and $\_COOKIE. ①$\_GET is used to obtain parameters in the URL, suitable for non-sensitive data transmission such as paging and filtering, but attention should be paid to input verification; ②$\_POST is used to receive sensitive data submitted by the form, such as login information, and it is necessary to prevent SQL injection and XSS attacks; ③$\_SERVER provides information about the server and script execution environment, such as the current script name, user IP and request method, and check whether the key exists before use; ④$\_SESSION is used to maintain user status across pages, and session\_st must be called first when using it.