Common problems and solutions for PHP variable scope include: 1. The global variable cannot be accessed within the function, and it needs to be passed in using the global keyword or parameter; 2. The static variable is declared with static, and it is only initialized once and the value is maintained between multiple calls; 3. Hyperglobal variables such as $_GET and $_POST can be used directly in any scope, but you need to pay attention to safe filtering; 4. Anonymous functions need to introduce parent scope variables through the use keyword, and when modifying external variables, you need to pass a reference. Mastering these rules can help avoid errors and improve code stability.

Mastering the commonly used operators of PHP can deal with most development scenarios, mainly including: 1. Arithmetic operators ( , -, , /, %) are used for mathematical calculations and support dynamic variable operations, but pay attention to the problems that may be caused by automatic type conversion; 2. Comparison operators (==, ===, !=, >

In PHP, there are mainly two ways to define constants: define() function and const keyword. define() is a runtime function that can dynamically define constants at any location; while const is a language structure processed in the compilation stage and must be used directly in global or in classes and cannot be placed in conditional statements, loops or functions. The difference between the two is mainly reflected in: 1. define() supports dynamic definition, suitable for situations determined based on configuration files; 2. const is suitable for use in class definition constants and namespaces, which are more readable and organized and have slightly better performance; 3. Const needs to pay attention to scope issues when defining constants, such as namespace prefixes that cannot be omitted; 4. Both do not support modifying defined values, but define()

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PHP multi-line comments use // format, suitable for commenting multiple lines of code or adding instructions. 1. It can be used to comment code segments during debugging, such as annotating database query code; 2. It is used to write functions or classes to describe functions and parameters; 3. It is used to add copyright information or file descriptions; precautions include: 4. Do not use multi-line comments in nesting, otherwise it will lead to parsing errors; 5. Avoid */ in strings resulting in misclosure; 6. Use shortcut keys with IDE to improve efficiency. Proper use of multi-line comments can help improve code readability and maintenance.

There are three common annotation methods in PHP, namely single-line comments, multi-line comments and document block comments. 1. Single-line comments use // or #, suitable for short descriptions, and can be placed at the end of the code line or a separate line; 2. Multi-line comments start with / and end with /, suitable for detailed descriptions such as function functions or version records; 3. Document block comments start with /**, combined with tags such as @param, @return, etc., can be recognized by IDE and tools for generating code prompts and documents. In addition, it is recommended to write comments for functions and classes, especially public methods; add interpretative comments in complex logic; avoid meaningless comments; and not submit commented debug code. Mastering these annotation methods and usage suggestions will help improve code readability and maintenance efficiency.

There are three ways to add comments in PHP: //, # and //, among which // is the most commonly used. Comments can improve code readability and maintenance, and should explain "why" rather than "what was done" while keeping it updated. Functions and classes should indicate functions and input and output parameters, and inline comments can be used to explain complex logic. The rational use of comments helps team collaboration and post-debugs to avoid excessive or outdated comments.

There are four common problems when installing PHP, namely: 1. PHP cannot start or report an error. You should check the php.ini configuration error and verify it with php-t; 2. When the installation of the extension fails, confirm whether the underlying dependency is fully installed; 3. The web server cannot parse the PHP file and needs to check whether PHP-FPM is enabled and match the server configuration; 4. For multiple version conflicts, you can view the current use version through whichphp and php-v and adjust the environment variables or switch tools. When encountering problems, you should gradually check the configuration and log to locate the cause.