PHP void

function identifier(): void {
    // execution block
}

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Syntactic Rules and Behavior

1. Implicit return: Reaching the end of the function block without encountering a return statement.
2. Explicit empty return: Using the return; statement without an accompanying expression.

// Valid: Implicit return
function executeTask(): void {
    $operation = 2 + 2;
}

// Valid: Explicit empty return
function terminateEarly(bool $condition): void {
    if ($condition) {
        return; 
    }
    $operation = 2 + 2;
}

// Invalid: Triggers Fatal error
function returnString(): void {
    return "string"; 
}

// Invalid: Triggers Fatal error
function returnNull(): void {
    return null; 
}

​

Type System Restrictions

• Scope of Usage: The void type is exclusively a return type. It cannot be used as a parameter type declaration or a class property type declaration. Attempting to do so will result in a parse error.
• Union and Intersection Types: void cannot be combined with other types. Declarations such as void|string or ?void are syntactically invalid. The void type inherently represents the absence of a value, making it mutually exclusive with any type that represents a value.

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Inheritance and Variance

• The child method must declare a void return type, or (as of PHP 8.1) it may declare a never return type. Because never is a bottom type in PHP’s type system, it is a valid subtype of void.
• A child class cannot widen the return type to omit void, nor can it change it to a specific value-bearing type like mixed, int, or null.

class ParentClass {
    public function process(): void {}
}

class ChildClass extends ParentClass {
    // Valid: Exact match
    public function process(): void {}
}

class SubtypeChildClass extends ParentClass {
    // Valid (PHP 8.1+): 'never' is a bottom type and a valid subtype of 'void'
    public function process(): never {
        throw new RuntimeException('Halted');
    }
}

class InvalidChildClass extends ParentClass {
    // Invalid: Fatal error (Declaration must be compatible)
    public function process(): int {
        return 1;
    }
}