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.