1. php
  2. /basics
  3. /constants

Introduction to PHP Constants


In PHP, constants are variables that have a fixed value and cannot be changed during the execution of a program.


There are several ways to define a constant in PHP:

  1. Using the define() function: This function takes two arguments, the first is the name of the constant and the second is the value to be assigned to it. For example:
define("MAX_USERS", 100);
  1. Using the const keyword: This keyword is used to define a constant in a class or at the global scope. For example:
class MyClass {
    const MAX_USERS = 100;
const MAX_USERS = 100;

Once a constant is defined, it can be accessed anywhere in the program by referencing its name without the use of the $ symbol. For example, to access the value of the MAX_USERS constant, you would use the following code:


Constants are case-sensitive, so "MAX_USERS" and "max_users" are two different constants.


// Define a constant using the define() function
define("MAX_USERS", 100);

// Define a constant using the const keyword
const MIN_USERS = 10;

// Define a constant using the final keyword
final class MyClass {
    final const MAX_ITEMS = 50;

// Access the constants
echo "Maximum number of users: " . MAX_USERS . "<br>";
echo "Minimum number of users: " . MIN_USERS . "<br>";
echo "Maximum number of items in MyClass: " . MyClass::MAX_ITEMS;


The output will be:

Maximum number of users: 100
Minimum number of users: 10
Maximum number of items in MyClass: 50

In this example, three constants are defined: "MAX_USERS", "MIN_USERS", and "MAX_ITEMS". They are defined using the define() function, the const keyword, and the final keyword respectively. The values of these constants can be accessed anywhere in the program by referencing their names.

Best Practices

  1. Use all uppercase letters for constant names. This is a common convention and makes it easier to identify constants in your code.

  2. Use underscores to separate words in constant names. For example, use "MAX_USERS" instead of "MAXUSERS"

  3. Use descriptive names for constants. This makes it easier to understand the purpose of the constant and its usage in the code.

  4. Use the define() function or the const keyword to define constants. These methods ensure that the constant cannot be reassigned a different value.

  5. Define constants at the top of your script or in a separate configuration file. This makes it easier to find and manage constants in your code.

  6. Avoid using magic numbers in your code. Instead, use constants to give meaning to specific values.

  7. Do not use constants to store sensitive data. Constants are stored in plain text and can be easily read by anyone with access to your code.

  8. Be careful when using constants in a class. If you use the const keyword, the constant will be shared across all instances of the class. If you want to have different values for different instances, use static properties.

  9. Constants should be defined with a value only once, if there is any attempt to redefine it, it will raise an error.

  10. Be careful when defining constants in a global scope, it might cause naming conflicts if the constant name is already defined by another library or package.