can not get x_username header in nginx/1.20.1

i sent x_username header with curl to my site and i see this header in tcpdump but nginx do not pass it to my php-fpm

        location ~ .php$ {
            try_files $uri =404;
            fastcgi_pass   127.0.0.1:9070;
            fastcgi_index index.php;
            fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
            include fastcgi_params;
        }

curl command

curl --location -k  --request POST 'https://xxxxx.com/test.php' 
--header 'Accept: application/json' 
--header 'Content-Type: application/x-www-form-urlencoded' 
--header 'x_username: xxxx' 
--data-urlencode 'login_username=xxxxxx'

nginx version is nginx/1.20.1

php version is 70

when i removed _ from header name and it worked but x_username not work ( xusername work )