Detailed introduction to variable knowledge in php

This article brings you a detailed introduction to the knowledge of variables in PHP. It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

The meaning of variables

A container used to store information, the amount that can change during the running of the program

Naming rules for variables
Variables start with the $ symbol
Variable names can only start with letters or underscores
Variable names can only be letters, underscores, numbers, and cannot have special characters: commas, periods, exclamation points, spaces, etc.
Variable names are case-sensitive

Variable naming method
a, small camel case, the first letter of the first word is lowercase, and the first letter of subsequent words is capitalized. $ userName (general user Variable, function, method names)
Big camel case, the first letter of each word is capitalized. $ UserModel (generally used for class and class file naming)
b, underline naming method, $ _user_name

Type of variable (3 major categories, 8 subcategories)
a, scalar (single value)
Boolean type boolean, the value is true or false. Check whether it is a Boolean type is_boolean()
Integer type integer. The unsigned integer value is twice as large as the signed integer value. The default declaration variable is signed. Check whether it is an integer type is_integer()
Float type float (double), detect whether it is a floating point type is_float()
String type string, is_string()
b, consistent with (multi-value: multi-value type value )
Array array, detect whether it is an array is_array()
Object object, detect whether it is an object is_object()
c, special
Resources generally need to be parsed with functions
Undefined, no value or NULL

Get the variable type
Function gettype($var) to get the variable Type

Conversion of variables
a, temporary conversion, just temporarily changes the type of the variable: the first way: (int), (bool), (float), (string ), (array), (object); The second way is intval(), floatval(), strval();
b, permanent conversion, permanent conversion of the variable type.settype($var)

Scope of variables (category)
1, global variables, created outside the function, can be accessed anywhere, within the function When accessing, you need to add $global statement or use $GLOBALS['name']
2, local variable, created within the function, can only be used inside the function, the function ends Destroyed when
3, static variable, is created within the function through the keyword static, can only be used inside the function, (acts on initialization) exists after the first declaration of the function , it is not destroyed after the function ends. When the function is accessed again, the last value is used directly
4, super global variable, can be accessed in all scopes of the script; classification
a,$ GLOBALS, a global combination array containing all variables, the variable name is the key name of the array
b,$ _SERVER, an array containing information such as header, path, script location, etc.
1)$ _SERVER['HTTP_HOST'] Request the Host content in the header information to obtain the current domain name
2)$ _SERVER["SERVER_ADDR"] The IP address of the server currently running the script.
c,$ _FILES Gets information about uploaded files
1)$ _FILES[‘myFile’][‘name’] The original name of the client file.
2)$ _FILES[‘myFile’][‘type’] The MIME type of the file, which requires the browser to provide support for this information, such as "image/gif".
3)$ _FILES[‘myFile’][‘size’] The size of the uploaded file, in bytes.
4)$ _FILES['myFile']['tmp_name'] The temporary file name stored on the server after the file is uploaded
d,$ _REQUEST,#P##OST,##_GET,#C##OOKIE,_SESSION

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