VariableVariables is a feature in PHP that uses variable values as another variable name. It uses $$var to achieve dynamic access to variables, process form input, and build flexible configuration structures. For example, $name="age"; echo$$name is equivalent to the output value of $age; common usage scenarios include: 1. Dynamic access to variables, such as ${$type.'_info'}, different variables can be selected according to the conditions; 2. Automatically assign values when processing form input, but attention should be paid to security risks; 3. Build a flexible configuration structure and obtain corresponding values through string names; when using it, you need to pay attention to code maintenance, naming conflicts and debugging difficulties. It is recommended that only

PHP has 8 variable types, commonly used include Integer, Float, String, Boolean, Array, Object, NULL and Resource. To view variable types, use the gettype() or is_type() series functions. PHP will automatically convert types, but it is recommended to use === to strictly compare the key logic. Manual conversion can be used for syntax such as (int), (string), etc., but be careful that information may be lost.

The yield keyword is used to create generators, generate values on demand, and save memory. 1. Replace return to generate finite sequences, such as Fibonacci sequences; 2. Implement infinite sequences, such as natural sequences; 3. Process big data or file readings, and process them line by line to avoid memory overflow; 4. Note that the generator can only traverse once, and can be called by next() or for loop.

The basic PHP syntax includes: 1. Use wrapping code; 2. Use echo or print to output content, where echo supports multiple parameters; 3. Variables do not need to declare types, start with $. Common types include strings, integers, floating-point numbers, booleans, arrays and objects. Mastering these key points can help you get started with PHP development quickly.

PHP variables start with $, and the naming must follow rules, such as they cannot start with numbers and are case sensitive; the scope of the variable is divided into local, global and hyperglobal; global variables can be accessed using global, but it is recommended to pass them with parameters; mutable variables and reference assignments should be used with caution. Variables are the basis for storing data, and correctly mastering their rules and mechanisms is crucial to development.

PHP files are server-side scripting language files used for dynamic web development. They can process form data, connect to databases, generate dynamic content, and control access rights. It ends with .php, and the code returns the result to the browser after it is executed on the server. To run PHP files, you need to install a local server environment such as XAMPP, put the files in the server directory and access them through the browser. PHP is usually mixed with HTML. It is recommended to master HTML, CSS, JavaScript and basic programming concepts before learning. Practice more to get started quickly.

Comments cannot be careless because they want to explain the reasons for the existence of the code rather than the functions, such as compatibility with old interfaces or third-party restrictions, otherwise people who read the code can only rely on guessing. The areas that must be commented include complex conditional judgments, special error handling logic, and temporary bypass restrictions. A more practical way to write comments is to select single-line comments or block comments based on the scene. Use document block comments to explain parameters and return values at the beginning of functions, classes, and files, and keep comments updated. For complex logic, you can add a line to the previous one to summarize the overall intention. At the same time, do not use comments to seal code, but use version control tools.

The key to writing PHP comments is clear, useful and concise. 1. Comments should explain the intention behind the code rather than just describing the code itself, such as explaining the logical purpose of complex conditional judgments; 2. Add comments to key scenarios such as magic values, old code compatibility, API interfaces, etc. to improve readability; 3. Avoid duplicate code content, keep it concise and specific, and use standard formats such as PHPDoc; 4. Comments should be updated synchronously with the code to ensure accuracy. Good comments should be thought from the perspective of others, reduce the cost of understanding, and become a code understanding navigation device.