PHP learning variable usage summary_PHP tutorial

1. Define constants define("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 name spaces. This means for example TRUE and $TRUE are different.

2. Ordinary variables $a = "hello";

3. Variable variables (using two dollar signs ($))
$$a = "world";
Both variables are defined:
The content of $a is "hello" and the content of $hello is "world".
Therefore, it can be expressed as:
echo "$a ${$a}"; or echo "$a $hello"; both of them will output: hello world
To use mutable variables for arrays, An ambiguity must be resolved. This is when writing $$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
static $a = 0 inside the function;
Note: Assigning it with 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 in the function body can be used outside the function. Global variables defined outside the function cannot be used inside the function body. To access variables in the global scope, you can use special PHP custom $ GLOBALS array:
For example: $GLOBALS["b"] = $GLOBALS["a"] + $GLOBALS["b"];
A real global variable imported with the global statement in a function domain is actually The above establishes a reference to the global variable
global $obj;
Note: The static and global definitions of variables are implemented in an application manner

6. Assign a value to the variable : Assignment by address (simple reference):
$bar = &$foo; //Add & sign before the variable to be assigned
Changing the new variable will affect the original variable. This assignment operation Faster
Note: Only named variables can be assigned by address
Note: If
$bar = &$a;
$bar = &$foo;
Changing the value of $bar only Can change the value of variable foo without changing the value of a (the reference changes)

7.PHP super global variable $GLOBALS: Contains a reference pointing to the global scope of each current script Valid variables within. The keys of this array are the names of global variables. The $GLOBALS array exists since PHP 3.
$_SERVER: Variables are set by the web server or 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 by the execution environment to the script.
$_REQUEST: Variables submitted to the script 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: Since PHP 4.3.0, the file information in $_FILES no longer exists in $_REQUEST.
$_SESSION: Variable currently registered for the script session.

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