Nullable types in functions in PHP - Step-by-Step Execution

Concept Flow - Nullable types in functions

Function Declaration

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Parameter Type: Nullable

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Function Call with Value

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Function Executes Normally

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Function Call with null

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Function Accepts null Without Error

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Return or Output

The function declares a parameter that can be a type or null. When called, it accepts either a value of that type or null, allowing flexible inputs.

Execution Sample

PHP

<?php
declare(strict_types=1);
function greet(?string $name) {
  if ($name === null) {
    return "Hello, Guest!";
  }
  return "Hello, $name!";
}

echo greet("Alice");
echo greet(null);

This function greets a person by name or says 'Hello, Guest!' if null is passed.

Execution Table

Step	Function Call	Parameter $name	Condition $name === null	Return Value	Output
1	greet("Alice")	"Alice"	False	"Hello, Alice!"	"Hello, Alice!"
2	greet(null)	null	True	"Hello, Guest!"	"Hello, Guest!"
3	End	-	-	-	No more calls

💡 No more function calls; execution ends.

Variable Tracker

Variable	Start	After Call 1	After Call 2	Final
$name	-	"Alice"	null	null or string

Key Moments - 2 Insights

Why can the function accept null even though the parameter type is string?

Because the parameter is declared as ?string, which means it can be a string or null, as shown in execution_table row 2 where $name is null and accepted.

What happens if we pass a non-string and non-null value?

PHP will throw a TypeError because the parameter expects either a string or null, not other types. This is not shown in the table but is important to remember.

Visual Quiz - 3 Questions

Look at the execution table, what is the return value when greet(null) is called?

A "Hello, Guest!"

B "Hello, null!"

C null

D Error

💡 Hint

Check execution_table row 2 under 'Return Value'

At which step does the condition $name === null evaluate to True?

A Step 1

B Step 3

C Step 2

D Never

💡 Hint

Look at the 'Condition $name === null' column in execution_table

If we remove the question mark from ?string in the function parameter, what would happen when calling greet(null)?

A Function returns "Hello, Guest!"

B PHP throws a TypeError

C Function returns "Hello, null!"

D Function treats null as empty string

💡 Hint

Nullable types allow null; removing ? disallows null, causing error on null input

Concept Snapshot

Nullable types in PHP functions:
- Use ? before type to allow null (e.g., ?string)
- Function accepts value of type or null
- Check for null inside function to handle special cases
- Passing null without ? causes TypeError
- Enables flexible and safe function inputs

Full Transcript

This example shows a PHP function greet with a nullable string parameter. The question mark before string means the parameter can be a string or null. When calling greet with "Alice", the function returns "Hello, Alice!". When calling greet with null, the function returns "Hello, Guest!" because it checks if the parameter is null. This allows the function to handle missing or optional names safely. If the question mark is removed, passing null would cause an error. This trace helps understand how nullable types work in PHP functions.