为什么nginx不读取php文件？ 403禁止错误

Question

Nginx在读取index.php文件时显示403 Forbidden错误。但是使用index.html文件它可以正常工作。我假设php的问题。这是我的配置：

    server {    
    listen   80;
    server_name  localhost;


    root   /usr/share/nginx/html;
    index index.php index.html index.htm;

    location / {
        try_files $uri $uri/ =404;
    }
    error_page 404 /404.html;
    error_page 500 502 503 504 /50x.html;
    location = /50x.html {
        root /usr/share/nginx/html;
    }

    location ~ \.php$ {
        try_files $uri =404;
        fastcgi_pass unix:/var/run/php-fpm/php-fpm.sock;
        fastcgi_index index.php;
        fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
        include fastcgi_params;
    }
}