Understanding PHP Variables
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 variables are the basis for writing dynamic websites, and understanding their usage and rules will make you more comfortable in the development process. Variables are used in PHP to store data, such as strings, numbers, arrays, etc. It is very flexible, but it is also easy to make mistakes without understanding its underlying mechanism.

Variable naming rules
The PHP variable must start with the dollar sign $
, followed by a legal variable name. There are some basic rules for variable names:
- Must begin with letters or
_
- Can't start with numbers
- Only include letters, numbers and underscores (Az, 0-9, _)
- Case sensitivity (e.g.
$name
and$Name
are two different variables)
Common errors such as writing: $123user
or $user-name
, these are illegal. Correct examples include: $userName
, $_age
, $user123
, etc.

Scope of variables
Variables are not accessible anywhere in the code. There are three main scopes in PHP: local scope, global scope and hyperglobal scope.
- Local variables: defined inside a function and can only be used in that function.
- Global variables: defined outside the function and cannot be accessed directly in the function unless the
global
keyword is used. - Hyperglobal variables: such as
$_GET
,$_POST
,$_SESSION
, etc., can be accessed anywhere in the script.
For example, if you want to modify a global variable in a function, you can do this:

$age = 25; function updateAge() { global $age; $age = 30; }
However, it should be noted that frequent use of global
may make the code difficult to maintain, and it is recommended to handle it through parameter passing.
Variable variables and reference assignments
PHP has a special function called "mutable variable", which means that the variable name can be the value of a variable. For example:
$name = "city"; $$name = "Beijing"; // equivalent to $city = "Beijing"; echo $city; // Output Beijing
Although this function is interesting, it is not very commonly used and can easily confuse others when reading the code, so it is recommended to use it with caution.
Another thing to note is reference assignment. Normal assignment is a copy value, while reference assignment refers to two variables pointing to the same memory address. For example:
$a = 10; $b = &$a; // Reference assignment $a = 20; echo $b; // Output 20
This operation will affect the values of multiple variables and is suitable for specific scenarios, but you should also be careful to cause bugs if you misoperate.
Basically that's it. Variables may seem simple, but in actual development, mastering their behavior and details is very critical to writing stable and efficient code.
The above is the detailed content of Understanding PHP Variables. For more information, please follow other related articles on the PHP Chinese website!

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