PHP use private method as callback

The second version works because preg_match_all is called from within the class scope and will execute the callback immediately.

But the add_filter function only adds the callback to the global $wp_filter array. The callbacks in that array are then called at a later stage from outside the class you defined the method in. Consequently, visibility rules apply making the callback inaccessible.

You can¹ get around this by wrapping the call to the method into an anonymous function, e.g.

public function __construct(){    add_filter(        'plugin_action_links',        function($links, $file) {            return $this->AddSettingsLink($links, $file);        },        10,        2    );}

However, this requires at least PHP 5.4 (see changelog in Manual) due to $this being unavailable in the anonymous function before that version.

Another option would be to have the SettingsHandler implement __invoke to turn it into a Functor, e.g. you add

public function __invoke($links, $file){    return $this->AddSettingsLink($links, $file);}

and change the ctor code to

add_filter('plugin_action_links', $this, 10, 2);

Since the class implements __invoke as a public entry point, the instance can be used as callback. This way, you can keep the private stuff private.

On a side note, I'd move the code adding the filter outside the class. Having it in the ctor makes the class less testable (yeah, I know no one tests WP plugins, but still). Instead, put it into your plugin file and then include everything else required from there, e.g.

// your-plugin.phpinclude 'SettingsHandler.php';add_filter('plugin_action_links', new SettingsHandler, 10, 2);

But that's up to you.

¹_{Note: apparently this doesn't work in Wordpress because it will try to serialize the closure somewhere along the way. In general, this a working pattern to provide private methods as callbacks.}