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 ):
The above is the detailed content of How Do Nullable Types Work in PHP 7 and Above?. For more information, please follow other related articles on the PHP Chinese website!

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