Union types in PHP - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of function with union type parameter

What is the output of this PHP code using union types?

PHP

<?php
function describe(mixed $value): string {
    if (is_int($value)) {
        return "Integer: $value";
    } elseif (is_string($value)) {
        return "String: $value";
    } else {
        return "Other type";
    }
}

// Call with union type int|string
function show(int|string $input): string {
    return describe($input);
}

echo show(10) . "\n";
echo show("hello") . "\n";
echo show(3.14) . "\n";

A

Integer: 10
String: hello
Fatal error: Uncaught TypeError

B

Fatal error: Uncaught TypeError
String: hello
Other type

C

Integer: 10
String: hello
String: 3.14

D

Integer: 10
String: hello
Other type

💡 Hint

Remember union types restrict accepted types. The third call uses a float which is not allowed.

The function show() accepts only int or string. The first two calls pass valid types and return expected strings. The third call passes a float, which is not allowed by the union type, causing a TypeError. However, since the code calls describe() with mixed, it returns 'Other type' for non-int/string values. But show() restricts input to int|string, so passing float causes a TypeError before describe() runs.

❓ Predict Output

intermediate

Return type union behavior

What will this PHP code output when run?

PHP

<?php
function getValue(bool $flag): int|string {
    if ($flag) {
        return 42;
    } else {
        return "forty-two";
    }
}

var_dump(getValue(true));
var_dump(getValue(false));

A

string(2) "42"
string(9) "forty-two"

B

int(42)
int(0)

C Fatal error: Return type must be int|string

D

int(42)
string(9) "forty-two"

💡 Hint

The function returns either an int or a string depending on the boolean input.

The function getValue declares a union return type int|string. When called with true, it returns the integer 42. When called with false, it returns the string 'forty-two'. Both are valid return types, so var_dump outputs the type and value accordingly.

🔧 Debug

advanced

Identify the error caused by union type mismatch

What error will this PHP code produce when executed?

PHP

<?php
function process(int|string $data): void {
    echo $data;
}

process(100);
process("test");
process(3.14);

A Parse error: syntax error, unexpected '3.14' (float literal)

B Fatal error: Uncaught TypeError: Argument 1 passed to process() must be int|string, float given

C

Output:
100
test
3.14

D Warning: echo expects parameter 1 to be string, float given

💡 Hint

Check the type of the third argument passed to the function.

The function process() accepts only int or string. The first two calls pass valid types. The third call passes a float (3.14), which is not allowed by the union type. This causes a TypeError at runtime, stopping execution with a fatal error.

📝 Syntax

advanced

Which union type declaration is valid?

Which of the following PHP function signatures correctly declares a union type parameter?

A function example(int|string $param): void {}

B function example(int | string $param): void {}

C function example(int |string|null $param): void {}

D function example(int|string| $param): void {}

💡 Hint

Check the spacing and syntax around the union type declaration.

Option A is correct syntax: no spaces around the pipe character. Option A has spaces around the pipe which is invalid syntax in PHP union types. Option A is invalid because null is not allowed without a question mark or explicit nullable type. Option A ends with a pipe which is a syntax error.

🚀 Application

expert

Determine the number of valid keys in union typed array

Given this PHP code, how many keys will the resulting array have?

PHP

<?php
/**
 * @param array<int|string, int|string> $input
 * @return array<int|string, int|string>
 */
function transform(array $input): array {
    $result = [];
    foreach ($input as $key => $value) {
        if (is_int($key) && $key % 2 === 0) {
            $result[$key] = (string)$value;
        } elseif (is_string($key) && strlen($key) > 3) {
            $result[$key] = (int)$value;
        }
    }
    return $result;
}

$input = [
    2 => 100,
    3 => 200,
    "name" => "1234",
    "id" => "5678",
    "longkey" => "42"
];

$output = transform($input);
echo count($output);

A 2

B 4

C 3

D 5

💡 Hint

Check which keys meet the conditions inside the loop.

The function adds to $result only keys that are even integers or strings longer than 3 characters. Keys 2 (even int), "name" (length 4), and "longkey" (length 7) qualify. Keys 3 (odd int) and "id" (length 2) do not. So the result has 3 keys.

Practice

1. What does a union type in PHP allow you to do?
function example(int|string $value) {}

easy

A. Only accept integer values

B. Accept multiple types for a parameter or return value

C. Only accept string values

D. Combine two functions into one

Solution

1. Step 1: Understand union type syntax

   The | symbol between types means the parameter can be either type.

2. Step 2: Analyze the example function

   The function accepts either an int or a string as input.

3. Final Answer:

   Accept multiple types for a parameter or return value -> Option B

4. Quick Check:

   Union types allow multiple types [OK]

Hint: Look for the | symbol to spot union types [OK]

Common Mistakes:

• Thinking union types combine functions
• Assuming only one type is allowed
• Confusing union types with type casting

2. Which of the following is the correct syntax for a union type in PHP 8+?
function test(?? $param) {}

easy

A. int|string, float $param

B. int&string $param

C. int|string&float $param

D. int|string $param

Solution

1. Step 1: Recall union type syntax

   Union types use the | symbol between types, e.g., int|string.

2. Step 2: Check each option

   int|string $param uses | correctly. int&string $param uses & which is intersection, not union. int|string&float $param mixes union and intersection incorrectly. int|string, float $param uses a comma which is invalid.

3. Final Answer:

   int|string $param -> Option D

4. Quick Check:

   Union types use | symbol [OK]

Hint: Union types always use | between types [OK]

Common Mistakes:

• Using & instead of | for union types
• Separating types with commas
• Mixing intersection and union syntax incorrectly

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

function showType(int|string $value) {
    if (is_int($value)) {
        return "Integer: $value";
    } else {
        return "String: $value";
    }
}

echo showType(10) . "\n";
echo showType("hello") . "\n";

medium

A. 10\nhello

B. String: 10\nInteger: hello

C. Integer: 10\nString: hello

D. Error: Type mismatch

Solution

1. Step 1: Understand the function behavior

   The function checks if the input is an integer or string and returns a formatted string accordingly.

2. Step 2: Trace the calls

   Calling with 10 (int) returns "Integer: 10". Calling with "hello" (string) returns "String: hello".

3. Final Answer:

   Integer: 10\nString: hello -> Option C

4. Quick Check:

   Type check returns correct output [OK]

Hint: Check type with is_int or is_string inside function [OK]

Common Mistakes:

• Assuming output is just values without labels
• Confusing string and int outputs
• Expecting a type error for valid union types

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

function add(int|float $a, int|float $b): int|float {
    return $a + $b;
}

add(5, "10");

medium

A. Passing a string "10" instead of int or float causes a type error

B. Return type should be only int

C. Union types cannot be used in return types

D. Function should not accept two parameters

Solution

1. Step 1: Check parameter types

   The function expects parameters of type int or float only.

2. Step 2: Analyze the function call

   The call passes 5 (int) and "10" (string). String is not allowed, causing a type error.

3. Final Answer:

   Passing a string "10" instead of int or float causes a type error -> Option A

4. Quick Check:

   Parameter types must match union types exactly [OK]

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

Common Mistakes:

• Assuming string '10' is accepted as int
• Thinking union types disallow return types
• Ignoring parameter count

5. You want a function that accepts either an array of integers or a single integer and returns the sum. Which union type and code is correct?

function sumValues(?? $input): int {
    if (is_array($input)) {
        return array_sum($input);
    }
    return $input;
}

hard

A. int|array $input

B. int|string $input

C. array|float $input

D. int|bool $input

Solution

1. Step 1: Identify input types needed

   The function accepts either a single integer or an array of integers.

2. Step 2: Match union type to input

   int|array $input uses int|array, which matches the requirement. Others include wrong types like string, float, or bool.

3. Final Answer:

   int|array $input -> Option A

4. Quick Check:

   Union type matches all accepted input types [OK]

Hint: Match union types exactly to all input types needed [OK]

Common Mistakes:

• Using string or bool instead of array
• Not including all input types in union
• Confusing float with int