PHP ignores the execution overhead of comments, because comments are discarded during the compilation stage and will not enter the opcode execution process; 2. The only negligible performance impact is the microsecond parsing time when the script is first loaded, and there is almost no impact after OPcache is enabled; 3. Priority should be paid to the real performance bottlenecks such as database queries and loops, rather than the number of comments.

Explain "why" rather than "what to do", such as skipping the CSV headline line; 2. Use less in-line comments and replace complex logic with clear function names; 3. Indicate edge cases, such as the fallback mailbox is GDPR compliant when it is empty; 4. Use PHPDoc to standardize public API parameters and exceptions; 5. Keep comments updated, outdated comments are worse than no comments.

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Do not write sensitive information (such as passwords, API keys) in comments, because it may be exposed by logs or version control, and you should use environment variables or key management tools instead; 2. Do not "annotate" outdated code with comments, which will cause confusion, and you should delete it directly and restore it by Git history, and explain the reason for the deletion if necessary; 3. Do not write obvious nonsense comments (such as "create empty arrays"), let the variable names be interpreted by themselves, and only explain "why" when the logic is complicated; 4. Do not leave large TODO/FIXME without responsible persons and deadlines, which are easy to become garbage. You should use project management tools to track them, or indicate the person in charge and deadline in comments.

When the code is not intuitive (such as bit operations and regularity) you must comment on the intention; 2. Public functions need to comment on the purpose and implicit logic (such as relying on holiday status); 3. Use TODO/FIXME to mark temporary plans or to-do items (such as hardcoded API addresses); 4. When citing external algorithms, the source (such as StackOverflow link); the core of the annotation is to reduce the cost of understanding, not to make up the numbers.

Use htmlspecialchars() and preprocessing statements to prevent XSS and SQL injection; 2. Verify input through trim(), length check and filter_var(); 3. Add honeypot field or reCAPTCHAv3 to resist spam comments; 4. Use CSRF token to ensure that the source of the form is trustworthy; 5. Use preprocessing statements during storage and HTMLPurifier to purify the content before display, and do not trust user input throughout the process, so as to build a safe PHP comment system.

PHPDoc is a JavaDoc-based PHP document standard. It uses special annotation blocks (/*.../) to add type and behavior information to the code without changing runtime behavior; 2. The core tags include @param (parameter type), @return (return value type), @var (variable/attribute type) and @throws (may throw exceptions), improving IDE intelligent prompts and static analysis capabilities; 3. Practical suggestions: Priority is given to the use of PHP native type declarations. PHPDoc is used for complex types such as array structures, keeping annotations accurate and updating synchronously with the code, and gradually adding @param and @return to new functions to develop habits, and ultimately implementing the self-documentation of the code and enhancing tool support.

Priority is given to the use of "why" comments rather than "what to do" comments, because the former provides background or business logic that the code cannot express; 2. Avoid duplication of content that has been clearly expressed by the code, and improve readability by improving variable or function naming; 3. Use PHPDoc block comments to explain the function function function, and keep inline comments focused on explaining the reasons for decision-making, thereby improving code maintainability and saving subsequent development time.