出现can't connect to MySQL server using '' (13)的错误，结果是 SELinux 不让 httpd 访问外网，一开始还以为是iptables造成的，关闭之后发现还是不行。
原因：
#getsebool -a | grep httpd
[root@centos]#getsebool -a | grep httpd
allow_httpd_anon_write --> off
allow_httpd_mod_auth_ntlm_winbind --> off
allow_httpd_mod_auth_pam --> off
allow_httpd_sys_script_anon_write --> off
httpd_builtin_scripting --> on
httpd_can_check_spam --> off
httpd_can_network_connect --> off
httpd_can_network_connect_cobbler --> off
httpd_can_network_connect_db --> off
httpd_can_network_memcache --> off
httpd_can_network_relay --> off
httpd_can_sendmail --> off
httpd_dbus_avahi --> on
httpd_enable_cgi --> on
httpd_enable_ftp_server --> off
httpd_enable_homedirs --> off
httpd_execmem --> off
httpd_read_user_content --> off
httpd_setrlimit --> off
httpd_ssi_exec --> off
httpd_tmp_exec --> off
httpd_tty_comm --> on
httpd_unified --> off
httpd_use_cifs --> off
httpd_use_gpg --> off
httpd_use_nfs --> off
发现 httpd_can_network_connect_db --> off
解决方案：
#/usr/sbin/setsebool -P httpd_can_network_connect_db on