Union types in PHP - Step-by-Step Execution

Concept Flow - Union types

Function with Union Type

↓

Call function with value

↓

Check if value matches any type

↓

Run function

↓

Return result

The function accepts values of multiple types. It checks the input type and runs if it matches any allowed type, else it errors.

Execution Sample

PHP

<?php
function example(int|string $value): string {
  return "Value is: $value";
}

echo example(10);
echo example("hello");

This code defines a function that accepts either an int or a string and returns a string showing the value.

Execution Table

Step	Function Call	Parameter Type	Type Check Result	Action	Output
1	example(10)	int	Matches int|string	Function runs	Value is: 10
2	example("hello")	string	Matches int|string	Function runs	Value is: hello
3	example(3.14)	float	Does not match int|string	Type error thrown	Fatal error

💡 Execution stops on type error when input does not match any union type.

Variable Tracker

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

Key Moments - 2 Insights

Why does example(3.14) cause an error?

Because 3.14 is a float and the function only accepts int or string, as shown in execution_table row 3.

Can the function accept both int and string in the same call?

No, each call passes one value. The union means the value can be either int or string, not both at once.

Visual Quiz - 3 Questions

Look at the execution table, what is the output when calling example(10)?

A "Value is: hello"

B Fatal error

C "Value is: 10"

D No output

💡 Hint

Check row 1 in the execution_table for output of example(10).

At which step does the type check fail?

A Step 3

B Step 2

C Step 1

D No failure

💡 Hint

Look at the 'Type Check Result' column in execution_table.

If the function accepted float as well, what would happen at step 3?

A Output is empty string

B Function runs and outputs value

C Type error still thrown

D Program crashes

💡 Hint

If float was added to union, step 3 would match and function would run.

Concept Snapshot

Union types allow a function parameter or return to accept multiple types.
Syntax: function foo(int|string $param): string
The function runs if input matches any type in the union.
If input type does not match, a type error occurs.
Useful for flexible but type-safe code.

Full Transcript

This example shows how PHP union types work. The function example accepts either an int or a string as input. When called with 10 (an int), it runs and returns 'Value is: 10'. When called with 'hello' (a string), it runs and returns 'Value is: hello'. But when called with 3.14 (a float), it causes a type error because float is not in the union. The execution table traces each call, showing the parameter type, type check result, action, and output. The variable tracker shows how the input variable changes with each call. Key moments clarify why the error happens and that the function accepts one type per call. The quiz tests understanding of outputs and type checks. The snapshot summarizes union types as a way to accept multiple types safely in PHP functions.