If you are stand on Root folder
your .htaccess should like
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteRule ^$ app/webroot/ [L]
RewriteRule (.*) app/webroot/$1 [L]
</IfModule>
If you are stand in to app folder
your .htaccess should like
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteRule ^$ webroot/ [L]
RewriteRule (.*) webroot/$1 [L]
</IfModule>