The problem I'm trying to solve is that some functions in WordPress have multiple signatures by means of either variadic arguments or use of func_get_args() (🤦 ).

For example the add_query_arg() function definition is:

function add_query_arg( ...$args ): string

In practice there are two allowable signatures:

function add_query_arg( string $key, string $value, string $url ): string
function add_query_arg( string[] $args, string $url ): string

The wpdb::prepare() method definition is:

public function prepare( string $query, ...$args ): string

In practice there are two allowable signatures:

public function prepare( string $query, string ...$args ): string
public function prepare( string $query, string[] $args ): string

In both cases the return type doesn't vary but there are two supported signatures. I would like PHPStan to flag an error if the function is called in a manner which doesn't match one of those signatures (for example if the first parameter of add_query_arg() is an array and more than two parameters are passed).

Clarification: stubs for WordPress are provided by the phpstan-wordpress extension.

Unfortunately the only thing you can do here is to write custom rules to detect these situations.