What Is Debugging?

You can debug by hand, reading code and adding log statements, or with a debugger: a tool that pauses a running program so you can step through it line by line and inspect its state. Debuggers work anywhere software runs, from a web server to the microcontroller in an embedded system. Developers use them to trace bugs in their own code. Quality assurance (QA) testers use debuggers too, pinpointing and reporting errors in the software they're testing so developers can fix them.

The debugging process

Debugging follows a repeatable sequence. IBM's overview of debugging breaks it into 6 steps:

1. Reproduce the conditions that trigger the bug, so you can watch it fail firsthand.
2. Locate the bug by reading the code and reviewing your error logs.
3. Determine the root cause, not just the symptom.
4. Fix the code.
5. Retest to confirm the fix works and hasn't introduced a new bug.
6. Document what caused the bug and how you fixed it, for the next person who hits it.

The order matters. A fix applied before you understand the root cause usually patches the symptom and leaves the bug in place.

Debugging vs. testing

Testing and debugging are different jobs, and they usually belong to different people:

• Testing proves that a defect exists. It's typically QA's work, and it ends with a bug report, not a fix.
• Debugging traces that defect to its root cause in the source code and resolves it. That's typically the developer's work.

Testing brackets debugging on both sides: tests surface the bug first, and tests validate the fix afterward.

Common types of bugs

Most bugs fall into 3 broad types:

• Syntax errors: a missing parenthesis or semicolon stops the code from running at all. They're usually quick to find, and our guide to fixing syntax errors in WordPress walks through the most common case.
• Logic errors: the code runs but produces the wrong result, like a cart that applies a 10% discount instead of 15%. Nothing crashes, which is exactly what makes them hard to spot.
• Runtime errors: failures that appear only while the program runs, such as a request for a file that doesn't exist or a script that runs out of memory.

A fourth type, the semantic error, describes code that runs but produces no meaningful output.

Debugging tools

Most debugging tools cost nothing:

• Browser DevTools: every major browser ships a built-in JavaScript debugger. Press F12 and check the Console for errors.
• Xdebug: the free, open source debugger for PHP, the language WordPress runs on.
• GDB: the GNU Project's free debugger for compiled languages such as C and C++.
• Error logs and print statements: no installation at all, and enough for many everyday bugs.

Paid debugging tools are a niche, mostly hardware probes for embedded devices. Nothing a website owner needs comes with a price tag.

How to debug a WordPress site

WordPress ships with a built-in debug mode. Add 3 lines to your site's wp-config.php file, above the line that reads /* That's all, stop editing! */:

• define( 'WP_DEBUG', true ); switches debug mode on.
• define( 'WP_DEBUG_LOG', true ); saves every PHP error, notice, and warning to wp-content/debug.log.
• define( 'WP_DEBUG_DISPLAY', false ); keeps those messages out of your visitors' browsers.

Reproduce the problem, then read debug.log: it names the exact file and line that raised each error. The WordPress Advanced Administration Handbook (updated July 7, 2025) recommends debug mode for staging and local installs only, so turn it off on a live site once you have your answer. Then fix what the log found, starting with our walkthrough of 18 common WordPress errors.