jsonSerialize Must Have Return Type

Until PHP 8.1, PHP would not enforce the type compatibility between a native interface and its custom implementation. This was for backward compatibility, and it is now over: PHP checks for type compatibility.

If making the returntype mixed or compatible is not possible at the moment, it is possible to use the ReturnTypeWillChange attribute to avoid this error message until it is actually fixed.

This affects all PHP native interfaces, and jsonSerialize is the most frequent to be reported.

PHP code

<?php

class x implements JsonSerializable {
     function __construct() { echo __METHOD__; }
     function jsonSerialize() {}
}

new x;

?>

Before

x::__construct

After

PHP Deprecated:  Return type of x::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice

Deprecated: Return type of x::jsonSerialize() should either be compatible with JsonSerializable::jsonSerialize(): mixed, or the #[\ReturnTypeWillChange] attribute should be used to temporarily suppress the notice
x::__construct

PHP version change

This behavior changed in 8.1

Error Messages

Analyzer