Why Union types in PHP? - Purpose & Use Cases

The Big Idea

What if your code could accept different types without messy checks and bugs?

The Scenario

Imagine you are writing a function that can accept either a number or a string. Without union types, you have to write separate functions or add lots of checks everywhere to handle each type manually.

The Problem

This manual approach is slow and error-prone because you must constantly check the type and write duplicate code. It's easy to forget a case or make mistakes, leading to bugs and confusing code.

The Solution

Union types let you declare that a function or variable can accept multiple types directly. This makes your code clearer, safer, and easier to maintain because the language itself knows what types are allowed.

Before vs After

✗ Before

function process($value) {
  if (is_int($value)) {
    // handle int
  } elseif (is_string($value)) {
    // handle string
  } else {
    throw new Exception('Invalid type');
  }
}

✓ After

function process(int|string $value) {
  // handle int or string directly
}

What It Enables

Union types enable writing flexible and robust code that clearly communicates what types are accepted, reducing bugs and improving readability.

Real Life Example

For example, a function that formats user input might accept either a string or an integer ID. Union types let you declare this clearly so the function works smoothly with both.

Key Takeaways

Manual type checks are slow and error-prone.

Union types let you accept multiple types cleanly.

This leads to clearer, safer, and easier-to-maintain code.

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