Enum backed values in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an enum backed value in PHP?

An enum backed value in PHP is a way to assign a specific scalar value (like int or string) to each enum case, allowing the enum to hold and return that value.

beginner

How do you declare a backed enum with string values in PHP?

Use the syntax: enum Status: string { case Active = 'active'; case Inactive = 'inactive'; } where : string declares the enum as backed by strings.

intermediate

What happens if you try to assign a non-scalar value to a backed enum case?

PHP will throw a fatal error because backed enum cases must have scalar values (int or string) only.

beginner

How can you get the backed value of an enum case in PHP?

You can access the backed value using the ->value property, for example: Status::Active->value returns 'active'.

intermediate

Can backed enums have methods in PHP?

Yes, backed enums can have methods just like regular enums, allowing you to add behavior related to the enum cases.

Which scalar types can be used for backed enum values in PHP?

A int and string

B float and bool

C array and object

D resource and null

Backed enums in PHP only support int and string scalar types for their values.

How do you declare an enum backed by integers in PHP?

A enum Status { case Active = 1; case Inactive = 0; }

B enum Status: string { case Active = '1'; case Inactive = '0'; }

C enum Status: int { case Active = 1; case Inactive = 0; }

D enum Status: float { case Active = 1.0; case Inactive = 0.0; }

You declare an int backed enum by specifying ': int' after the enum name and assigning integer values to cases.

What will Status::Active->value return if enum Status: string { case Active = 'active'; }?

A 'active'

B Status::Active

C null

D An error

The ->value property returns the backed value assigned to the enum case, which is 'active'.

Can you assign a float value to a backed enum case in PHP?

A Only if you use a backed enum of type float

B Yes, float is allowed

C Only if you cast it to string

D No, only int and string are allowed

PHP backed enums only support int and string scalar types, float is not allowed.

What will happen if you try to assign an array as a backed enum value?

A It will assign the first element

B Fatal error

C It will be ignored

D It will convert to string

Backed enum values must be scalar (int or string). Assigning an array causes a fatal error.

Explain what a backed enum is and how you use it in PHP.

Think about enums that hold specific values like numbers or words.

Describe the rules and limitations for values assigned to backed enum cases in PHP.

Consider what types PHP allows for enum case values.

Practice

1.

What is the main purpose of using backed enums in PHP?

easy

A. To allow enums to inherit from classes

B. To link enum cases to fixed strings or numbers

C. To enable enums to have methods

D. To make enums mutable

Solution

1. Step 1: Understand enum backed values

   Backed enums associate each enum case with a fixed string or integer value.

2. Step 2: Identify the purpose

   This linking helps in storing, comparing, or displaying enum data easily.

3. Final Answer:

   To link enum cases to fixed strings or numbers -> Option B

4. Quick Check:

   Backed enums link cases to values = A [OK]

Hint: Backed enums always connect cases to fixed values [OK]

Common Mistakes:

• Thinking enums can inherit classes
• Confusing backed enums with mutable enums
• Assuming enums cannot have methods

2.

Which of the following is the correct syntax to declare a backed enum with string values in PHP?

enum Status: string {
    case ACTIVE = 'active';
    case INACTIVE = 'inactive';
}

easy

A. enum Status: string { case ACTIVE = 'active'; case INACTIVE = 'inactive'; }

B. enum Status { case ACTIVE = 'active'; case INACTIVE = 'inactive'; }

C. enum Status: int { case ACTIVE = 'active'; case INACTIVE = 'inactive'; }

D. enum Status: string { ACTIVE = 'active'; INACTIVE = 'inactive'; }

Solution

1. Step 1: Check enum declaration syntax

   Backed enums require specifying the type after the enum name, e.g., : string.

2. Step 2: Verify case declarations

   Each case must use the case keyword and assign a value with =.

3. Final Answer:

   enum Status: string { case ACTIVE = 'active'; case INACTIVE = 'inactive'; } -> Option A

4. Quick Check:

   Backed enum syntax with type and cases = B [OK]

Hint: Backed enums need type after enum name and case keyword [OK]

Common Mistakes:

• Omitting the type after enum name
• Not using 'case' keyword before cases
• Using wrong type like int for string values

3.

What will be the output of the following PHP code?

enum Color: int {
    case RED = 1;
    case GREEN = 2;
    case BLUE = 3;
}

echo Color::from(2)->name;

medium

A. Color::GREEN

B. 2

C. GREEN

D. Error: No case for value 2

Solution

1. Step 1: Understand Color::from(2)

   The from() method returns the enum case matching the value 2, which is GREEN.

2. Step 2: Access the name property

   Using ->name returns the case name as a string, here "GREEN".

3. Final Answer:

   GREEN -> Option C

4. Quick Check:

   from(2) returns GREEN name = GREEN [OK]

Hint: from(value) returns enum case; ->name gives case name [OK]

Common Mistakes:

• Printing the value instead of the name
• Confusing from() with tryFrom()
• Expecting full enum class name output

4.

Identify the error in the following PHP enum backed values code:

enum Status: string {
    case PENDING = 'pending';
    case COMPLETED = 'completed';
}

$status = Status::from('done');

medium

A. No error, code runs fine

B. Compile error: Enum must use int type

C. Syntax error: Missing semicolon after enum cases

D. Runtime error: Value 'done' not found in enum cases

Solution

1. Step 1: Check the from() method usage

   The from() method throws an error if the value does not match any enum case.

2. Step 2: Analyze the value 'done'

   Since 'done' is not a defined case value, a runtime error occurs.

3. Final Answer:

   Runtime error: Value 'done' not found in enum cases -> Option D

4. Quick Check:

   from() with invalid value causes runtime error = D [OK]

Hint: from() fails if value not in enum cases [OK]

Common Mistakes:

• Assuming from() returns null on failure
• Thinking syntax error due to semicolons
• Believing enum must be int type only

5.

Given the following enum, which code snippet correctly converts a string value to an enum case safely without throwing an error?

enum Direction: string {
    case NORTH = 'north';
    case SOUTH = 'south';
    case EAST = 'east';
    case WEST = 'west';
}

Choose the correct usage:

hard

A. $dir = Direction::tryFrom('east');

B. $dir = Direction::from('southwest');

C. $dir = Direction::from('up');

D. $dir = Direction::tryFrom('down');

Solution

1. Step 1: Understand from() vs tryFrom()

   from() throws error if value not found; tryFrom() returns null instead.

2. Step 2: Check each option's value

   Only tryFrom('east') matches a valid case and safely returns the enum case.

3. Final Answer:

   $dir = Direction::tryFrom('east'); -> Option A

4. Quick Check:

   tryFrom() safely converts valid value = C [OK]

Hint: Use tryFrom() to avoid errors on invalid values [OK]

Common Mistakes:

• Using from() with invalid values causing errors
• Expecting tryFrom() to throw errors
• Confusing enum case names with values