How to execute a PHP function before Wordpress generates a page?

Question

WordPress

0. Follow the link ( www.site.io || www.site.io/dir )
2. PHP function is executed
3. Wordpress generates the page
It is desirable to do this without hacking the engine

Answer 1

add_action("init",function(){
//здесь то, что нужно сделать
});

enter in the functions.php of your theme. you can google the firing order of events in wordpress, init is the very first