How to fix a ‘Object of class WP_Error could not be converted to string’ error in WordPress

If you see a blank page while trying to log in to your WordPress site, check your web server’s error logs. You may get the following error:

stderr: PHP Catchable fatal error:  Object of class WP_Error
could not be converted to string in
/var/[PATH TO YOUR DOCUMENT ROOT]/wp-includes/default-constants.php on line 139

Note that the line number may be different depending on your version of WordPress but the code generating the error is as follows:

function wp_plugin_directory_constants() {
    if ( !defined('WP_CONTENT_URL') )
        define( 'WP_CONTENT_URL', get_option('siteurl') . '/wp-content');…

Do not be tempted to debug by editing the code as it’s not the source of the error. Your problem will very likely be that the siteurl option value in your WordPress database does not contain a valid entry. In this case, the error message is telling you that get_option() receives a WP_Error object rather than a string that it’s expecting.

To fix this:

1. First check the siteurl option to verify that the value is indeed incorrect. Run the following SQL:
   SELECT * FROM `wp_options` WHERE option_name = 'siteurl';

   You will likely find a serialized array containing a WP_Error object.

2. Correct the option value by setting it with your domain’s URL:
   UPDATE wp_options SET option_value = '[YOUR URL]' WHERE option_name = 'siteurl';

I’m not sure what overwrites the siteurl option value. Most likely there is a misbehaving plugin installed or malware has infected your installation. Be sure to run a scan on your server.