Impossible to write on file in php with nginx + php-fpm

Question

I can't understand why a php function is_writable() returning always false. I see nothing in the log about it, and after searching for hours : I can't understand why.

File setup :

[root@localhost owncloud]# ls -la /etc/test
-rwxrwxrwx. 1 nginx nginx 4 Jul 31 17:41 /etc/test

test.php

<?php
    $filename = '/etc/test';
    echo exec('whoami');
    if (file_exists($filename)) {
        echo 'Exist';
    } else {
        echo 'NOT exist';
    }
    if (is_writable($filename)) {
        echo 'writable';
    } else {
        echo 'NOT writable';
    }
?>

it returns :

nginx
Exist
NOT writable 

My nginx has the following configuration :

/etc/nginx/nginx.conf

user  nginx;
worker_processes  1;

error_log  /var/log/nginx/error.log;
#error_log  /var/log/nginx/error.log  notice;
#error_log  /var/log/nginx/error.log  info;

pid        /run/nginx.pid;

events {
    worker_connections  1024;
}

http {
    include       /etc/nginx/mime.types;
    default_type  application/octet-stream;
    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                  '$status $body_bytes_sent "$http_referer" '
                  '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;
    sendfile        on;
    keepalive_timeout  65;

   server {

    listen 8080 ssl;
    ssl_certificate     /etc/nginx/certs/owncloud.crt;
    ssl_certificate_key /etc/nginx/certs/owncloud.key;

    error_log /var/log/nginx/owncloud.log;
    access_log /var/log/nginx/owncloud.log;

    server_name owncloud;

    root /var/www/owncloud;

    client_max_body_size 10G; # set max upload size
    fastcgi_buffers 64 4K;

    rewrite ^/caldav(.*)$ /remote.php/caldav$1 redirect;
    rewrite ^/carddav(.*)$ /remote.php/carddav$1 redirect;
    rewrite ^/webdav(.*)$ /remote.php/webdav$1 redirect;

    index index.php;
    error_page 403 = /core/templates/403.php;
    error_page 404 = /core/templates/404.php;

    location = /robots.txt {
        allow all;
        log_not_found off;
        access_log off;
    }

    location ~ ^/(data|config|\.ht|db_structure\.xml|README) {
        #allow instead of deny for testing purpose    
            allow all;
    }

    location / {
            rewrite ^/.well-known/host-meta /public.php?service=host-meta last;
            rewrite ^/.well-known/host-meta.json /public.php?service=host-meta-json last;
            rewrite ^/.well-known/carddav /remote.php/carddav/ redirect;
            rewrite ^/.well-known/caldav /remote.php/caldav/ redirect;
            rewrite ^(/core/doc/[^\/]+/)$ $1/index.html;

            try_files $uri $uri/ index.php;
    }

    location ~ ^(.+?\.php)(/.*)?$ {
            try_files $1 = 404;

            include fastcgi_params;
            fastcgi_param SCRIPT_FILENAME $document_root$1;
            fastcgi_param PATH_INFO $2;
            fastcgi_param HTTPS on;
            fastcgi_pass 127.0.0.1:9000;
    }
  }
}

/etc/php-fpm.conf

include=/etc/php-fpm.d/www.conf
[global]
pid = /run/php-fpm/php-fpm.pid
error_log = /var/log/php-fpm/error.log
log_level = debug
daemonize = no

/etc/php-fpm.d/www.conf

[www]
listen = 127.0.0.1:9000
listen.allowed_clients = 127.0.0.1
listen.owner = nginx 
listen.group = nginx
user = nginx
group = nginx
pm = dynamic
pm.max_children = 50
pm.start_servers = 5
pm.min_spare_servers = 5
pm.max_spare_servers = 35
slowlog = /var/log/php-fpm/www-slow.log
php_admin_value[error_log] = /var/log/php-fpm/www-error.log
php_admin_flag[log_errors] = on
php_value[session.save_handler] = files
php_value[session.save_path] = /var/lib/php/session

Answer 1

The problem was SELinux that seems to prevent nginx to write to some files. Disabling SELinux solve the problem