Narrow $callback parameter type for add_shortcode()

[IanDelMar]

$callback is narrowed based on its invocation in core (see shortcodes.php#L432–L434):

$content = isset( $m[5] ) ? $m[5] : null;
$output  = $m[1] . call_user_func( $shortcode_tags[ $tag ], $attr, $content, $tag ) . $m[6];

Here, $shortcode_tags[$tag] is the $callback passed to add_shortcode(), $attr (often called $atts in callbacks) are the parsed attributes, $content is the enclosed content or null when no closing tag is present, and $tag is the shortcode name.
Accordingly, $callback is narrowed to: callable(array<string>, ?string, string): string.

Afaik, in current core, $attr is always an array; attributes without explicit values (e.g. [mytag attribute]) produce integer keys. As this is user input - though deprecated - keys may be int|string; hence array<string> and not array<string, string>.

[szepeviktor]

Very thorough.