PHP learning summary variables

This article shares with you the knowledge about the variables in PHP learning. Friends in need can refer to it

Variable identifier

The identifier is the name of the variable , PHP has the following rules for variable identifiers:

• consists of any letters, numbers, and underscores, but cannot start with a number

• Strictly case-sensitive

In order to scientifically standardize identifiers, there are mainly the following popular naming methods:

1. Camel case nomenclature

A word begins with a lowercase letter and the first letter of each subsequent word is capitalized (small camel case); or the first letter of each word is capitalized (large camel case, also known as Pascal nomenclature).

$myFirstName = 'jochen'; # 小驼峰法
$MyFirstName = 'jochen'; # 大驼峰法

2. Hungarian nomenclature

Variable name = attribute + type + object description.

$sMyFirstName = 'jochen';

3. Underline nomenclature

All letters are lowercase, and each word is separated by an underscore

$my_first_name = 'jochen';

Variable assignment

PHP provides two There are two ways to assign values ​​to variables, namely assignment by value and assignment by reference.

The most commonly used is assignment by value. When the value of an expression is assigned to a variable, the value of the entire original expression is assigned to the target variable. This means that when the value of one variable is assigned to another variable, changing the value of one variable will not affect the other variable.

Reference assignment is an alias for the content of the original variable. It is different from the pointer in C language, but changing the value of the variable will also affect the original variable.

$foo = 'jochen'; # 传值赋值
$bar = &$foo;    # 引用赋值

$bar = 'coco';   # $foo、$bar 的值都为 coco

Variable type

The variable type refers to the data type that can be saved in the variable. PHP's data types can be divided into three types: scalar data types, composite data types and special data types. PHP scalar data types include boolean (Boolean) , integer (integer) , float (floating point, also called double), string (string) , composite data types include array (array) , object (object) and callable (callable) , special data type resource(resource) and NULL(no type).

PHP is a weakly typed language. Variables do not need to declare their type before use, and the same variable can store multiple different types of data. Sometimes, in order to determine the data type of the current variable, we can use the following method to determine:

gettype()                 — 获取变量的类型
is_int() 或 is_integer()  — 检测变量是否是整数
is_numeric()              — 检测变量是否为数字或数字字符串
is_float() 或 is_double() — 检测变量是否是浮点型
is_string()               — 检测变量是否是字符串
is_bool()                 — 检测变量是否是布尔型 
is_array()                — 检测变量是否是数组
is_object()               — 检测变量是否是一个对象
is_null()                 — 检测变量是否为 NULL 
is_resource()             — 检测变量是否为资源类型

Sometimes, we need to force a variable to be evaluated as a certain type, and we must perform type cast:

(int) 或 (integer)  — 转换为整形
(bool) 或 (boolean) — 转换为布尔类型
(float) 或 (double) — 转换为浮点型
(string)            — 转换为字符串
(array)             — 转换为数组
(object)            — 转换为对象
(unset)             — 转换为 NULL

Variable scope

1. Super global variable

A variable that is predefined by PHP and is always available in any scope of any script. Superglobal variables include:

• $GLOBALS

• ##$_SERVER

• $_REQUEST

• $_POST

• $_GET

• ##$_FILES

##$_ENV
$_COOKIE
$_SESSION

• where
$GLOBALS

is an associative array that contains all global variables. The name of the variable is the

key of the array, and the value of the variable is the value of the array. 2. Global variables

Variables created outside the function, in

for

statements,

while statements, foreach statements The created variables are also global variables, their scope is from the statement that creates the variable to the end of the file, but they are not visible inside the function. It should be noted that once a constant is created, it will be visible globally and also within the function. 3. Local variables

A variable created inside a function has its scope from the statement that creates the variable to the end of the function. It should be noted that if the variable inside the function has the same name as the external global variable, the internal variable will overwrite the external variable. In addition, static variables created inside a function cannot be used outside the function.

require

and

include do not affect the scope, that is, if these two statements act inside the function, the variable has local scope; if they act outside the function, the variable Has global scope. Use the keyword global to declare a variable, which can be used to manually specify that a variable defined or used in a function has a global scope.

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