How Do Nullable Types Work in PHP 7 and Above?

Understanding PHP 7's Nullable Types (?string or ?int)

In PHP 7, a new feature known as "nullable types" was introduced. It allows developers to specify that a parameter or return value can be either the specified type or null. Nullable types are denoted by a question mark (?) placed before the type declaration.

Nullable Types in Parameters

Syntax:

function test(?string $parameter1, string $parameter2) {}

• Parameters marked as nullable (?string) can accept both strings and null values.
• Parameters without nullable types (string) must receive a non-null value or an error will be thrown.

Example:

test("foo", "bar"); // OK
test(null, "foo"); // OK
test("foo", null); // Error

Nullable Types in Return Values

Syntax:

function error_func(): int {
    return null ; // Error: Return value must be of type integer
}

function valid_func(): ?int {
    return null ; // OK
}

• Functions with nullable return types can return either the specified type or null.
• Functions without nullable return types must return non-null values or an error will be thrown.

Nullable Types in Properties (PHP 7.4 )

Syntax:

class Foo
{
    private ?object $bar = null; // OK: can be null
}

• Class properties can have nullable types, indicating that they can contain null values.

Nullable Union Types (PHP 8.0 )

As of PHP 8.0, "?T notation is considered a shorthand for the common case of T|null".

Syntax:

class Foo
{
    private object|null $baz = null;
}

• Nullable union types allow variables to be assigned either the specified type or null.

Error Handling

If the PHP version used is lower than 7.1, a syntax error will be thrown if nullable types are used. In such cases, remove the question mark (?).

References

• Nullable Type (PHP 7.1 ):
• Class Properties Type Declarations (PHP 7.4 ):
• Nullable Union Type (PHP 8.0 ):

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