How to define variables and constants in PHP

PHP is a widely used programming language with excellent scalability and practicality. In PHP, variables and constants are two very important concepts. They can be used to store and represent values ​​and store important information. In this article, we will introduce in detail how to define variables and constants in PHP to help beginners get started quickly.

1. Define variables

A variable is a name or identifier used to store a value. In PHP, the definition of variables can be divided into three steps: variable declaration, variable assignment and variable use. Let’s introduce it in detail below:

1.1 Declaration of variables

In PHP, the declaration of variables uses the $ symbol. Variable names must start with a letter or an underscore and must follow PHP's variable naming rules. The following are some rules about variable naming:

• Variable names must start with the $ sign.
• Variable names must start with letters or underscores, not numbers.
• Variable names can only contain letters, numbers and underscores.
• Variable names are case-sensitive, $name and $Name are different variable names.

The following is an example of variable declaration:

$name; // 变量$name被声明为一个未定义的变量

1.2 Assignment of variables

Assignment of variables uses the assignment operator (=). PHP automatically determines the variable type. The following is an example of variable assignment:

$name = "张三"; // 变量$name被赋值为值"张三"
$age = 18; // 变量$age被赋值为值18

1.3 Using variables

When using variables, you only need to add the $ symbol before the variable name. The following are examples of using variables:

echo $name; // 输出："张三"
echo $age; // 输出：18

2. Define constants

Constants refer to values ​​that will not change during the running of the PHP program. They are used to store important, frequently used or sensitive information. In PHP, constants are defined using the define() function. The following are the steps to define constants:

2.1 Define constants

define() function accepts two parameters: the constant name and its value. Constant names must start with a letter or an underscore, not a number. Constant names must also follow PHP's naming rules. The following is an example of defining a constant:

define("MY_CONST", "hello world"); // 定义常量MY_CONST，并给它赋值为"hello world"

2.2 Simple application

When using constants, there is no need to add the $ sign. The following is an example of using constants:

echo MY_CONST; // 输出："hello world"

2.3 Optional parameters of constants

define() function also has two optional parameters: the first is a Boolean value indicating whether to modify the constant name Case sensitive; the second is a Boolean value indicating whether the constant is to be defined globally. The following is an example of case sensitivity using the first optional parameter:

define("MY_CONST", "hello world", true); // 定义常量MY_CONST，并给它赋值为"hello world"

In this case, when accessing the constant, the constant name is not case sensitive:

echo my_const; // 输出："hello world"
echo MY_CONST; // 输出："hello world"

3. Summary

In PHP, variables and constants are two important concepts. They are used to store and represent values ​​and store important, commonly used or sensitive information. In this article, we detail the steps for defining variables and constants in PHP and provide some examples and usage of optional parameters. We hope that this article can help beginners master the basic use of variables and constants in PHP.

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