Please ask string concatenation

Consult string concatenation
What I want to achieve is that this button is pressed down if cat_id;?> takes this ID value, assuming 6
Then open http://localhost/myweb/wp-admin/post-new.php?cat=6

That's what I wrote:
onclick= "window.location.href= './wp-admin/post-new.php?cat= cat_id;?> '"

The ID value is also taken, but is always not spliced, and the result access has always been:
http://localhost/myweb/wp-admin/post-new.php?cat=


How is this place written to achieve this HTML and PHP hybrid splicing?

------to solve the idea----------------------

onclick= "window.location.href= './wp-admin/post-new.php?cat=
 
  cat_id;?> '"

Switch

onclick= "window.location.href= './wp-admin/post-new.php?cat=
 
  cat_id;?> '"

