Today I learned PHP knowledge about constants and data types
Here's what you'll learn today:
One, constant 1. Constant definition
A constant is an "identifier" in which the data stored is not and should not be changed . In general, there are two ways to define a constant:
// The first of these methods // define ("Constant name", "Constant Value"); Define ("HH","123"); // The second method of // Const Constant name = constant value;const 123;
Methods for the value of constants: two methods
// use the name directly, or take a value from the constant () function // directly using its name echo HH; // HH = 123 // using the constant ("constant name") function, note: The solid name is a string Echo constant ("HH"); // hh=123
Constants have the following characteristics:
1. Constants do not need to be used for $.
2. The value of a constant cannot be changed or destroyed.
3 A Hyper-global scope.
4. Constants can only store scalars.
In a project that is developed by multiple people, how do you tell if a constant exists or is defined by someone else?
Here, we give the function, this function is specifically to determine whether the constant exists:
Defined ("Constant")
Returns true if it exists, false if it does not exist
In PHP, if you use an undefined constant, you will get an error, but the constant will still have a value.
Reserved for constants
Predefined constants are pre-defined constants in the system, we can take them directly, with a constant, there are hundreds of, we need to know a few can be:
M_PI: pi;
Php_os: The operating system where PHP is running.
Php_version: This is the version number of PHP.
The largest integer of the php_int_max:php species.
Magic Constants
_file_: Represents the full physical path of the current Web page file
_dir_: Represents the folder where the current Web page file resides
_line_: Represents the line number where the current constant name is located
Ii. Types of data
There are 8 types of data in PHP altogether.
4 Types of basic type (scalar type):
Integral type: int integer
Floating point: Float,double,real
Strings: String
Boolean type: Bool,boolean
Two kinds of composite types:
Arrays: Array ()
Objects: Object ()
2 Types of special type
NULL type: null
Resource type: Resource
Method of data Type 1. Integer: int integer 1). There are four types of notation for integer types
123; // decimal 0123; // octal 0x123; // 16 binary $n 4 = 0b1010; // 2 binary
2). 10 binary conversion to other binary methods
Decbin (): The result returns a string of 2 binary digits of the number
decoct (): The result returns a string of 8 binary digits of the number
Dechex (): The result is a number that returns a string in the form of a binary number
3). Other binary conversions to 10 binary
Bindec ( string ): The result returns the number of the 2 binary numeric string corresponding to the ten -binary
Octdec ( string ): The result returns the number of the 8 binary numeric string corresponding to the ten -Binary
Hexdec ( string ): The result returns the number of the string corresponding to the Binary numbers
2. Floating point numberfloating-point numbers are written in two ways:
General notation: with decimal points. (123.456)
Scientific counting method: with special symbol E. (123456E3)
Note : Floating-point numbers should not be compared in size because all the final forms of the digits are binary, and the binary of most floating-point numbers is not allowed to be expressed. When an integer's result exceeds the maximum range of integers, it becomes a floating-point number.
3. Stringthe four-medium form of a string:
= = = <<<= <<< ' identifier b ' string content ... identifier B;
Four differences:
3. Boolean type (Bool,boolean)
it has only 2 data:true,false;
A common application scenario for Boolean types is to judge a variable directly.
0, 0.0, "", "0", NULL, Array (), false, and another is "Undefined variable "
Iv. Type Conversion Free conversion
in any operation, if a type of data is required, and the data given is not of that type, an automatic conversion usually occurs: converting the type to the type required by the target.
Forced conversions
An automatic type conversion is determined by a statement of "operator" or similar operator. Forcing type conversions is just a simple syntax:
(int), (float), (string), (bool), (array), (object)
The coercion type conversion above does not alter the variable's own data or type. Corresponding, there is a syntax that directly alters the data (and type):
Settype ($ variable name , "target type");
v. Common functions related to data types
Var_dump (): Print information about a variable
GetType ($ variable): Gets the data type, the return value is a string of the type name
SetType ($ variable name, "target Type"): Sets the data type of the variable, directly changes the data (and type)
Is_* (): Determine if a variable is of a certain type
Isset (): Determines whether the variable is null or exists
Empty (): Determines whether a variable is empty "", "0", 0,false,arry (), NULL is considered null, returns a Boolean value
Today's notes are written here and will continue tomorrow.
PHP Learning Day4---constants and data types