Union types in practice in PHP - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a union type in PHP?

A union type allows a function or method to accept multiple different types for a parameter or return value, separated by a pipe (|). For example, int|string means the value can be either an integer or a string.

beginner

How do you declare a function parameter that accepts either an int or a string in PHP?

You declare it using a union type like this: <code>function example(int|string $param) { ... }</code>. This means <code>$param</code> can be an integer or a string.

intermediate

Can union types include null in PHP? How?

Yes, you can include null in union types by adding it explicitly, for example: int|string|null. This means the value can be an integer, a string, or null.

intermediate

What happens if a value passed to a union typed parameter does not match any of the allowed types?

PHP will throw a TypeError at runtime, indicating the value does not match the declared union types.

beginner

Show an example of a function with a union return type in PHP.

Example:<br><code>function getId(): int|string {<br>  return rand(0,1) ? 123 : 'abc';<br>}</code><br>This function returns either an integer or a string.

Which of the following is a valid union type declaration for a parameter that accepts int or string?

A function foo(int|string& $param) {}

B function foo(int&string $param) {}

C function foo(int|string $param) {}

D function foo(int|string, $param) {}

Union types use the pipe (|) symbol between types, so int|string is correct.

What will happen if you pass a float to a parameter declared as int|string?

A It will be accepted without error.

B PHP will throw a TypeError at runtime.

C PHP will convert the float to int automatically.

D The function will return null.

Only int or string are allowed. Passing a float causes a TypeError.

How do you include null as an allowed type in a union type?

A Use <code>int|string|null</code>

B Use <code>int|string?null</code>

C Use <code>int|string|void</code>

D Use <code>int|string|nullable</code>

You explicitly add null to the union types.

Can union types be used for return types in PHP?

A Only for class types.

B No, union types only work for parameters.

C Only for void return types.

D Yes, you can declare multiple return types using union types.

Union types can be used for both parameters and return types.

Which PHP version introduced union types?

A PHP 8.0

B PHP 8.1

C PHP 7.4

D PHP 8.2

Union types were introduced in PHP 8.0.

Explain what union types are and how you use them in PHP function parameters.

Think about allowing more than one type for a parameter.

Describe how PHP handles a value passed to a union typed parameter that does not match any allowed type.

What happens if the type is wrong?

Practice

1. What does a union type declaration like int|string mean in PHP?

easy

A. The value must be both an integer and a string at the same time.

B. The value can only be a string.

C. The value can only be an integer.

D. The value can be either an integer or a string.

Solution

1. Step 1: Understand union type syntax

   The | symbol means the value can be one of the listed types.

2. Step 2: Interpret int|string

   This means the value can be either an integer or a string, not both simultaneously.

3. Final Answer:

   The value can be either an integer or a string. -> Option D

4. Quick Check:

   Union type int|string means either int or string [OK]

Hint: Union types allow multiple possible types separated by | [OK]

Common Mistakes:

• Thinking the value must be both types at once
• Confusing union with intersection types
• Assuming only one type is allowed

2. Which of the following is the correct syntax for an optional function parameter that accepts either an array or null using union types?

easy

A. function example(array|null $data = null) {}

B. function example(array&null $data) {}

C. function example(array? $data) {}

D. function example(array|null $data) {}

Solution

1. Step 1: Identify correct union type syntax

   Union types use | between types, so array|null is correct.

2. Step 2: Check default value for nullable parameter

   To accept null, the parameter should default to null, so $data = null is needed.

3. Final Answer:

   function example(array|null $data = null) {} -> Option A

4. Quick Check:

   Union type with default null = correct syntax [OK]

Hint: Use | for union and default null for optional nullable [OK]

Common Mistakes:

• Using & instead of | for union
• Missing default null for nullable parameter
• Using ? without type for union

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

function describe(mixed $value): string {
    return match (gettype($value)) {
        'integer' => 'Number: ' . $value,
        'string' => 'Text: ' . $value,
        default => 'Unknown type'
    };
}

echo describe(10) . ', ' . describe('hello') . ', ' . describe(3.14);

medium

A. Number: 10, Text: hello, Unknown type

B. Number: 10, Text: hello, Number: 3.14

C. Unknown type, Text: hello, Unknown type

D. Number: 10, Unknown type, Unknown type

Solution

1. Step 1: Analyze the match expression with gettype()

   For 10, gettype returns 'integer', so output is 'Number: 10'. For 'hello', gettype returns 'string', so output is 'Text: hello'. For 3.14, gettype returns 'double', which matches default case 'Unknown type'.

2. Step 2: Combine outputs

   Outputs joined by commas: 'Number: 10, Text: hello, Unknown type'.

3. Final Answer:

   Number: 10, Text: hello, Unknown type -> Option A

4. Quick Check:

   Match on gettype gives expected strings [OK]

Hint: Match gettype results carefully for each input [OK]

Common Mistakes:

• Assuming float is treated as integer
• Ignoring default case in match
• Confusing gettype return values

4. Identify the error in this PHP function using union types:

function process(int|string $input): int {
    if (is_string($input)) {
        return strlen($input);
    } else {
        return $input + 1;
    }
}

process(true);

medium

A. The is_string check is incorrect syntax.

B. The function should return string, not int.

C. Passing a boolean to a parameter expecting int|string causes a TypeError.

D. The function does not handle float inputs.

Solution

1. Step 1: Check parameter type declaration

   The function expects int|string, so only integers or strings are allowed.

2. Step 2: Analyze the call with true

   Passing true (boolean) violates the declared union type, causing a TypeError at runtime.

3. Final Answer:

   Passing a boolean to a parameter expecting int|string causes a TypeError. -> Option C

4. Quick Check:

   Type mismatch causes error [OK]

Hint: Check argument types match union types exactly [OK]

Common Mistakes:

• Ignoring type mismatch errors
• Thinking is_string is invalid syntax
• Assuming function return type is wrong

5. You want to write a function that accepts either a string or an array of strings and returns a single string. Which of the following implementations correctly uses union types and handles both cases?

hard

A. function joinStrings(string|array $input): string {
return implode(', ', $input);
}

B. function joinStrings(string|array $input): string {
if (is_array($input)) {
return implode(', ', $input);
}
return $input;
}

C. function joinStrings(string|array $input): string {
if (is_string($input)) {
return implode(', ', $input);
}
return $input;
}

D. function joinStrings(array|string $input): string {
if (is_array($input)) {
return $input;
}
return implode(', ', $input);
}

Solution

1. Step 1: Check union type declaration

   Both string|array and array|string are valid syntax.

2. Step 2: Verify handling of each type

   function joinStrings(string|array $input): string { if (is_array($input)) { return implode(', ', $input); } return $input; } checks if input is array, then joins with implode. If input is string, returns it directly. This correctly handles both cases.

3. Step 3: Identify errors in other options

   function joinStrings(string|array $input): string { return implode(', ', $input); } tries to implode without checking type, causing error if input is string. function joinStrings(string|array $input): string { if (is_string($input)) { return implode(', ', $input); } return $input; } incorrectly calls implode on string input. function joinStrings(array|string $input): string { if (is_array($input)) { return $input; } return implode(', ', $input); } returns array directly without joining.

4. Final Answer:

   function joinStrings(string|array $input): string { if (is_array($input)) { return implode(', ', $input); } return $input; } -> Option B

5. Quick Check:

   Check type then handle accordingly [OK]

Hint: Check type before processing union inputs [OK]

Common Mistakes:

• Calling implode on string input
• Returning array instead of string
• Not checking input type before processing