Try this:

if ($rule_0 = ""){
rewrite ^/api$ /api.php last;
}
if ($rule_1 = ""){
rewrite ^/admin/(.*) /admin/index.php last;
}
if (!-f $request_filename){
set $rule_2 1$rule_2;
}
if (!-d $request_filename){
set $rule_2 2$rule_2;
}
if ($rule_2 = "21"){
rewrite /. /index.php last;
}