Debugging in WordPress by Andrew Nacin

“Friends don’t let friends develop without WP_DEBUG.” —Andrew Nacin

Nacin gave a fantastic talk about Debugging in WordPress at WordCamp San Francisco 2011. I’ve heard more than one developer highly recommend this talk as a way to get started with debugging.

Debug Constants for Your Testing Environment

define( 'WP_DEBUG', true ); // Show all notices and errors
define( 'WP_DEBUG_DISPLAY', null ); // errors on, errors off, let php.ini decide
define( 'WP_DEBUG_LOG', true ); // Log errors to wp-content/error.log
define( 'SCRIPT_DEBUG', true ); // De-minify admin CSS and Javascript
define( 'SAVEQUERIES', true ); // DO NOT run this in production

Plugins

Common PHP Functions

Tracking Things Down

  • Step 1 – Disable plugins and switch to the default theme.
  • Check cron, rewrites, roles & capabilities.
  • Go to Settings ? Permalink to flush rewrites.
  • Are query vars set properly?
  • Does the SQL look right?
  • Rewrite rule matched?
  • Funny redirect? Check canonical redirects with
    remove_action( 'template_redirect', 'redirect_canonical' );
  • Testing multisite? Setup multisite and switch to multisite any time with define( 'MULTISITE', true ); to wp-config.php.
  • Dig into the codebase with phpxref, grep, ack.
  • Learn the stack.
  • Learn what things call what.

Common Traps

  • Make sure your code is actually running with die( 'wtf' ); or error_log();
  • White screen on the frontend with no errors, go to Appearance ? Themes to reset the theme.
  • Internal server errors? Check home and siteurl, then .htaccess, then canonical.
  • Moved or missing widgets? Make sure the sidebars have IDs.
  • Slow site with lots of pages? Check permalink structure (pre-3.3).
  • Make sure there are no extra “Template:” calls in style.css.
  • Strange admin behavior. Check table structure, step through upgrades by major release.
  • bbPress rewrites fail after activation. Re-register your post types before you flush_rewrite_rules();

Local Development

  • Map a domain to 127.0.0.1 using /etc/hosts
  • Run a staging server by replacing links with an output buffer by adding to wp-config.php
ob_start( 'switch_to_dev_urls' );
function switch_to_dev_urls( $buffer ) {
$live = 'http://EXAMPLE.COM';
$dev = 'http://DEV.EXAMPLE.COM';
return str_replace( $live, $dev, $buffer );
}

More

  • Xdebug for backtraces and profiling
  • KCacheGrind visualization
  • Use an IDE
  • Unit testing
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