Constants in classes in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constant in a PHP class?

A constant in a PHP class is a fixed value that cannot be changed once set. It is defined using the <code>const</code> keyword and is accessed without creating an object.

beginner

How do you define a constant inside a PHP class?

Use the <code>const</code> keyword followed by the constant name and value. For example: <code>const PI = 3.14;</code>

beginner

How do you access a class constant from outside the class?

Use the class name followed by the scope resolution operator <code>::</code> and the constant name. For example: <code>ClassName::CONSTANT_NAME</code>

beginner

Can class constants be changed after they are defined?

No, class constants cannot be changed after they are defined. They are fixed values throughout the program.

intermediate

What is the difference between a class constant and a class property?

A class constant is a fixed value defined with <code>const</code> and cannot change. A class property is a variable that can hold different values and can be changed.

How do you define a constant inside a PHP class?

A Using the <code>var</code> keyword

B Using the <code>const</code> keyword

C Using the <code>static</code> keyword

D Using the <code>define()</code> function

Class constants are defined using the const keyword inside the class.

How do you access a class constant from outside the class?

A Using <code>ClassName::CONSTANT</code>

B Using <code>$object->CONSTANT</code>

C Using <code>$object::CONSTANT</code>

D Using <code>ClassName->CONSTANT</code>

Class constants are accessed with the class name and the scope resolution operator ::.

Can you change the value of a class constant after it is set?

A Yes, anytime

B Only inside the class constructor

C No, constants are immutable

D Only if declared as <code>static</code>

Class constants cannot be changed once defined; they are immutable.

Which keyword is NOT used to define a class constant?

A var

B const

C define

D static

var is used for properties, not constants.

What is the correct way to define a constant named VERSION with value '1.0' inside a class?

A define('VERSION', '1.0');

B static $VERSION = '1.0';

C var VERSION = '1.0';

D const VERSION = '1.0';

Class constants are defined with const followed by the name and value.

Explain how to define and access a constant inside a PHP class.

Think about how constants are fixed and how you call them without creating an object.

Describe the difference between a class constant and a class property in PHP.

Consider mutability and how you use each in a class.

Practice

1. What keyword is used to declare a constant inside a PHP class?

easy

A. let

B. var

C. const

D. static

Solution

1. Step 1: Understand constant declaration in PHP classes

   In PHP, constants inside classes are declared using the const keyword, which defines a fixed value that cannot change.

2. Step 2: Differentiate from other keywords

   var is used for properties (deprecated), let is not a PHP keyword, and static is for static properties or methods, not constants.

3. Final Answer:

   const -> Option C

4. Quick Check:

   Class constants use const keyword [OK]

Hint: Use 'const' to declare fixed values in classes [OK]

Common Mistakes:

• Using 'var' instead of 'const'
• Trying to use 'let' which is not PHP
• Confusing 'static' with constants

2. Which of the following is the correct syntax to access a class constant named VERSION in PHP?

easy

A. ClassName::VERSION

B. $obj->VERSION

C. ClassName->VERSION

D. $obj:VERSION

Solution

1. Step 1: Recall how to access class constants

   Class constants are accessed using the scope resolution operator :: with the class name, like ClassName::CONSTANT.

2. Step 2: Identify incorrect options

   $obj->VERSION tries to access a property, not a constant; ClassName->VERSION is invalid syntax; $obj:VERSION uses single colon which is invalid syntax.

3. Final Answer:

   ClassName::VERSION -> Option A

4. Quick Check:

   Access constants with ClassName::CONSTANT [OK]

Hint: Use ClassName::CONSTANT to access class constants [OK]

Common Mistakes:

• Using object operator -> instead of ::
• Trying to access constants as properties
• Confusing variable and class name usage

3. What will be the output of the following PHP code?

class Car {
    const WHEELS = 4;
}
echo Car::WHEELS;

medium

A. 4

B. WHEELS

C. Error: Undefined constant

D. null

Solution

1. Step 1: Identify the constant declaration

   The class Car declares a constant WHEELS with value 4.

2. Step 2: Understand the echo statement

   The code prints the value of Car::WHEELS, which is 4.

3. Final Answer:

   4 -> Option A

4. Quick Check:

   Class constant value printed = 4 [OK]

Hint: ClassName::CONSTANT prints the constant's value [OK]

Common Mistakes:

• Expecting the name 'WHEELS' to print
• Thinking it causes an error
• Confusing constants with variables

4. What is wrong with this PHP code?

class Book {
    const TITLE = 'PHP Basics';
}

$book = new Book();
echo $book::TITLE;

medium

A. Cannot access constant via object instance with ::

B. Constant TITLE is not declared correctly

C. Missing semicolon after constant declaration

D. Code will output 'PHP Basics' without error

Solution

1. Step 1: Verify constant declaration

   The class Book correctly declares const TITLE = 'PHP Basics';.

2. Step 2: Verify access syntax

   $book = new Book(); creates an instance, and $book::TITLE is valid syntax to access the class constant from an instance. The code outputs PHP Basics without error.

3. Final Answer:

   Code will output 'PHP Basics' without error -> Option D

4. Quick Check:

   $instance::CONSTANT is valid [OK]

Hint: ClassName::CONSTANT or $instance::CONSTANT both valid [OK]

Common Mistakes:

• Using -> for constants instead of ::
• Declaring with var instead of const
• Thinking $instance::CONSTANT is invalid

5. Given the class below, which code correctly prints the constant STATUS without creating an object?

class Server {
    const STATUS = 'Online';
}

hard

A. echo Server->STATUS;

B. echo Server::STATUS;

C. echo $server::STATUS;

D. echo Server::status;

Solution

1. Step 1: Identify correct constant access syntax

   Constants are accessed using the class name and the scope resolution operator :: with the exact constant name in uppercase.

2. Step 2: Evaluate each option

   Server->STATUS is invalid because -> is for object properties; $server::STATUS is invalid because $server is undefined and constants should be accessed via class name; Server::status is wrong due to case sensitivity; only Server::STATUS is correct.

3. Final Answer:

   echo Server::STATUS; -> Option B

4. Quick Check:

   Use ClassName::CONSTANT with exact case [OK]

Hint: Use ClassName::CONSTANT with exact uppercase name [OK]

Common Mistakes:

• Using object operator -> instead of ::
• Ignoring case sensitivity in constant names
• Trying to access constants via undefined variables