Union types in practice in PHP - Step-by-Step Execution

Concept Flow - Union types in practice

Function with Union Type Parameter

↓

Call function with int

→Accept int

↓

Call function with string

→Accept string

↓

Call function with other type

→Error: Type Mismatch

↓

Function returns value

The function accepts either int or string as input. It checks the type and processes accordingly. If input is not int or string, it causes a type error.

Execution Sample

PHP

<?php
function printValue(int|string $value): void {
  echo "Value: $value\n";
}
printValue(10);
printValue("hello");
printValue(3.14); // error
?>

This code defines a function that accepts either an int or a string and prints it. It is called with int, string, and a float (which causes an error).

Execution Table

Step	Function Call	Parameter Type	Accepted?	Output or Error
1	printValue(10)	int	Yes	Value: 10
2	printValue("hello")	string	Yes	Value: hello
3	printValue(3.14)	float	No	TypeError: Argument 1 passed to printValue() must be of type int|string, float given

💡 Execution stops at step 3 due to TypeError caused by passing float instead of int|string.

Variable Tracker

Variable	Start	After Call 1	After Call 2	After Call 3
$value	undefined	10 (int)	"hello" (string)	3.14 (float) - error

Key Moments - 2 Insights

Why does passing 3.14 cause an error even though it is a number?

The function only accepts int or string as per the union type int|string. Float is not allowed, so step 3 in the execution_table shows a TypeError.

Can the function accept both int and string without separate overloads?

Yes, union types let the function accept either type in one definition, as shown in steps 1 and 2 where both int and string inputs are accepted.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output when printValue is called with "hello"?

A Value: hello

B Value: 10

C TypeError

D No output

💡 Hint

Check step 2 in the execution_table where printValue("hello") is called.

At which step does the function call cause a TypeError?

A Step 1

B Step 3

C Step 2

D No error

💡 Hint

Look at the 'Accepted?' and 'Output or Error' columns in the execution_table.

If the function parameter type was changed to int|float, what would happen at step 3?

A It would accept only int

B It would cause a TypeError

C It would accept 3.14 and print it

D It would accept string too

💡 Hint

Consider the union type change and how it affects the accepted parameter types.

Concept Snapshot

Union types allow a function parameter or return to accept multiple types.
Syntax: function f(int|string $param): void
The function accepts either int or string.
Passing other types causes a TypeError.
Useful for flexible but type-safe code.

Full Transcript

This example shows a PHP function using union types to accept either an int or a string parameter. The function printValue prints the value passed. When called with 10 (int) or "hello" (string), it works fine and prints the value. When called with 3.14 (float), it causes a TypeError because float is not part of the union type int|string. The execution table traces each call, showing accepted types and outputs or errors. The variable tracker shows how the parameter changes each call. Key moments clarify why float causes an error and how union types allow multiple accepted types in one function. The quiz tests understanding of outputs, error steps, and effects of changing union types. The snapshot summarizes union types as a way to accept multiple types safely in PHP functions.