set_error_handler and set_exception_handler do not necessarily return a callable

But just an array, depending on the callable used and the way it was called.
e.g. 2 examples where we don't get a callable but an array (and therefore get an error):

<?php

function accept_callable(callable $arg) {}

class Foo {
        public function __construct() {
                set_error_handler([__CLASS__, 'log_error']);
                set_exception_handler([$this, 'log_exception']);
        }

        public function log_error( $type, $message, $file, $line ) {
                echo "log error" . PHP_EOL;
                return true;
        }

        public function log_exception( $e ) {
                echo "log exception" . PHP_EOL;
                var_dump($e);
        }
}

$foo = new Foo();

$previousHandler = set_error_handler(static fn () => false);
restore_error_handler();

accept_callable($previousHandler);

https://3v4l.org/rjRSm

<?php

function accept_callable(callable $arg) {}

class Foo {
        public function __construct() {
                set_error_handler([$this, 'log_error']);
                set_exception_handler([$this, 'log_exception']);
        }

        private function log_error( $type, $message, $file, $line ) {
                echo "log error" . PHP_EOL;
                return true;
        }

        public function log_exception( $e ) {
                echo "log exception" . PHP_EOL;
                var_dump($e);
        }
}

$foo = new Foo();

$previousHandler = set_error_handler(static fn () => false);
restore_error_handler();

accept_callable($previousHandler);

https://3v4l.org/B7Et7

If you look at the php-src you see that the return type isn't MAY_BE_CALLABLE either, so this is not a bug in PHP but an issue in the docs only.

Start with the PHP documentation entries for set_error_handler() and set_exception_handler(), then compare their documented return values with the two examples and the linked reproductions. Done means the documentation accurately explains that the previous handler may be returned as an array rather than a callable in these cases.