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.

When writing PHP comments, you should clarify the purpose, logic and structure. 1. Each function and class uses DocBlock format to explain the role, parameters and return values; 2. Explain "why" in the key logic rather than just "what was done"; 3. Add a brief description at the top of the file, including functions, dependencies and usage scenarios; 4. Avoid nonsense comments, add only necessary instructions before complex logic, and do not record the modification history. This improves code readability and maintenance efficiency.

Defining constants in PHP, const is more suitable for constant definitions inside classes, and define() is more flexible and suitable for global or dynamic definitions. 1.const is a language structure, and must be a compile-time constant expression when defined, which is suitable for class or global namespaces; define() is a function, and the value can be the result of runtime calculation. 2.const is affected by the namespace, and the constants defined by define() are visible globally by default. 3. The const structure is clear and the IDE is good, which is suitable for object-oriented design; define() has high flexibility but may have higher maintenance costs. 4. define() supports runtime condition judgment and dynamic definition, but const does not support it. Therefore, class-related constants preferentially use co

PHP comparison operators need to pay attention to type conversion issues. 1. Use == to compare values only, and type conversion will be performed, such as 1=="1" is true; 2. Use === to require the same value as the type, such as 1==="1" is false; 3. Size comparison can be used on values and strings, such as "apple"

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.

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.