Variables and variable definitions in PHP_PHP tutorial

A brief description of PHP variables and definitions of constants, define, ordinary variables, static variables, assignment by address (simple reference), PHP super global variable $GLOBALS and other usages.

Variables are used to store values, such as numbers, text strings, or arrays.

Once a variable is set, we can use it repeatedly in the script.

All variables in PHP start with the $ symbol.

The correct way to set variables in PHP is:

$var_name = value; Beginners to PHP often forget the $ sign in front of the variable. If you do that, the variable will be invalid.

Let’s try to create a variable that holds a string, and a variable that holds a numeric value:

Example 1

Predefined variables
PHP provides a large number of predefined variables. Detailed documentation is not available as many variables depend on the version and settings of the running server, among other factors. Some predefined variables do not take effect when PHP is run from the command line.

Variable scope
The scope of a variable is the context scope in which it is defined (that is, its effective scope). Most PHP variables have only a single scope. This single scope span also includes files introduced by include and require.

PHP’s global variables are a little different from C language. In C language, global variables automatically take effect in functions unless overridden by local variables. Local variables in PHP will not be overwritten by global variables. If used, they will be the default initial values ​​of the variables. This may cause some problems, someone may accidentally change a global variable. Global variables in PHP must be declared global when used in functions.

Use static variables
Another important feature of variable scope is static variables. Static variables only exist in the local function scope, but their values ​​are not lost when program execution leaves this scope.

Note:
Naming rules for variables
Variable names must start with a letter or underscore "_".
Variable names can contain only alphanumeric characters and underscores.
Variable names cannot contain spaces. If the variable name consists of multiple words, they should be separated by underscores (such as $my_string) or start with a capital letter (such as $myString).

php variable definition

1. Define constantsdefine("CONSTANT", "Hello world.");
Constants can only contain scalar data (boolean, integer, float and string).
When calling a constant, you only need to simply use the name to get the value of the constant, without adding the "$" symbol, such as: echo CONSTANT;
Note: Constants and (global) variables are in different namespaces. This means for example TRUE and $TRUE are different.
2. Ordinary variables$a = "hello";
3. Variable variable (use two dollar signs ($))
$$a = "world";
Both variables are defined:
The content of $a is "hello" and the content of $hello is "world".
Therefore, it can be expressed as:
echo "$a ${$a}"; or echo "$a $hello"; they will both output: hello world
To use mutable variables with arrays, an ambiguity must be resolved. This is when writing $$a[1], the parser needs to know whether it wants $a[1] as a variable, or whether it wants $$a as a variable and extracts the variable with index [1] value. The syntax to solve this problem is to use ${$a[1]} for the first case and ${$a}[1] for the second case.
4. Static variables
Inside the function static $a = 0;
Note: Assigning the result of an expression in the declaration will cause parsing errors such as static $a =3+3; (error)
Static variables only exist in the local function domain (inside the function). After the function is executed, the variable value will not be lost and can be used for recursive calls
5. Global variables
Global variables defined within the function body can be used outside the function body. Global variables defined outside the function body cannot be used within the function body. To access variables in the global scope, you can use the special PHP custom $GLOBALS array:
For example: $GLOBALS["b"] = $GLOBALS["a"] + $GLOBALS["b"];
A real global variable imported with the global statement within a function scope actually establishes a reference to the global variable
global $obj;
Note: The static and global definitions of variables are implemented in the form of application
6. Assign a value to a variable : Pass address assignment (simple reference):
$bar = &$foo; //Add & symbol before the variable to be assigned
Changing the new variable will affect the original variable, and this assignment operation is faster
Note: Only named variables can be assigned by address
NOTE: If
$bar = &$a;
$bar = &$foo;
Changing the value of $bar can only change the value of variable foo, but not the value of a (the reference has changed)
7.PHP superglobal variable $GLOBALS: Contains a reference to variables that are valid in the global scope of each current script. The keys of this array are labeled with the names of global variables. The $GLOBALS array exists since PHP 3.
$_SERVER: Variables are set by the web server or are directly associated with the execution environment of the current script. Similar to the old $HTTP_SERVER_VARS array (still valid, but deprecated).
$_GET: Variables submitted to the script via the HTTP GET method.
$_POST: Variables submitted to the script via the HTTP POST method.
$_COOKIE: Variable submitted to the script via the HTTP Cookies method.
$_FILES: Variables submitted to the script via HTTP POST file upload.
The file upload form must have enctype="multipart/form-data"
$_ENV: Variables submitted to the script by the execution environment.
$_REQUEST: Variables submitted to scripts via GET, POST and COOKIE mechanisms, so this array is not trustworthy. The presence, absence, and order of all variables contained in this array are defined according to the variables_order configuration directive in php.ini. This array does not directly emulate earlier versions of PHP 4.1.0. See import_request_variables().
NOTE: As of PHP 4.3.0, file information in $_FILES is no longer present in $_REQUEST.
$_SESSION: The variable currently registered for the script session.