How to debug error establishing a database connection with WordPress

Question

When I try to access my WordPress site as localhost, I get:

Error establishing a database connection`.

How can I debug this problem? Where are logs I can see?

I am able to login to MySQL with user wordpressuser on localhost. I seem to have correct grants for wordpressuser on database wordpress:

root@myhost# mysql -hlocalhost -uwordpressuser -pmypassword 
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 109
Server version: 5.5.37-0+wheezy1 (Debian)

Copyright (c) 2000, 2014, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> show grants for wordpressuser@localhost;
+----------------------------------------------------------------------------------------------------------------------+
| Grants for wordpressuser@localhost                                                                                   |
+----------------------------------------------------------------------------------------------------------------------+
| GRANT USAGE ON *.* TO 'wordpressuser'@'localhost' IDENTIFIED BY PASSWORD '*somepassword' |
| GRANT ALL PRIVILEGES ON `wordpress`.* TO 'wordpressuser'@'localhost'                                                 |
+----------------------------------------------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

But when I run mysqlaccess, I get:

root@myhost# mysqlaccess localhost wordpressuser wordpress -pmypassword
mysqlaccess Version 2.06, 20 Dec 2000
By RUG-AIV, by Yves Carlier ([email protected])
Changes by Steve Harvey ([email protected])
This software comes with ABSOLUTELY NO WARRANTY.
Password for MySQL user wordpressuser: 

Sorry,
An error occured when trying to connect to the database
with the grant-tables:
* Maybe YOU do not have READ-access to this database?
* If you used the -U option, you may have supplied an invalid username?
  for the superuser?
* If you used the -U option, it may be possible you have to supply
  a superuser-password to, with the -P option?
* If you used the -P option, you may have supplied an invalid password?

wget localhost fails with connection refused:

user@myhost:~$ wget localhost
--2014-06-08 01:11:29--  http://localhost/
Resolving localhost (localhost)... ::1, 127.0.0.1
Connecting to localhost (localhost)|::1|:80... failed: Connection refused.
Connecting to localhost (localhost)|127.0.0.1|:80... connected.
HTTP request sent, awaiting response... 500 Internal Server Error
2014-06-08 01:11:29 ERROR 500: Internal Server Error.

My wp-config does seem to have the proper credentials (DB_NAME, DB_USER, DB_PASSWORD, DB_HOST).

How can I debug this problem?

Answer 1

I managed to temporarily work around the issue as follows:

Trying to find the place where php accesses the mysql database, I did a a

grep Ri db.user /var/www

to find that

/var/www/wp-includes/load.php

creates the database in this line:

$wpdb = new wpdb( DB_USER, DB_PASSWORD, DB_NAME, DB_HOST );

I tried hard-coding the values for DB_USER. DB_PASSWORD, DB_NAME, DB_HOST, and the site does load on the localhost now. Clearly, there was a mismatch between the values defined in wp-config.php, and those that actually get passed to new wpdb(...), though I haven't been able to track that down further yet.

Unlike what was suggested in one of the comments:

1. This was not a problem with the web-server. The site works normally now, from within localhost or from outside.

2. The command wget localhost does work correctly now. It downloads an index.html which contains the wordpress setup page