wp-config.php is the PHP file WordPress loads before it starts its normal bootstrap. It stores database connection details and can define constants that change the site’s URLs, debugging behavior, memory requests, filesystem paths, caching bootstrap, and authentication secrets.
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Constants must be declared before the file’s /* That's all, stop editing! Happy blogging. */ marker. WordPress core establishes defaults only when a value has not already been defined, so a correctly placed definition takes effect early enough to control startup.
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A standard installation uses wp-config.php for the database name, user, password, host, table prefix and authentication keys. It is also the site’s main environment switchboard. A constant is a PHP value that WordPress reads throughout a request; unlike a dashboard option, it is normally changed by editing the file or by deployment tooling.
Because the file runs before WordPress finishes loading, a single incorrect value can affect every page. A malformed URL can cause redirect loops, a visible debug notice can disclose server paths, and an unavailable memory setting can leave an administrator believing a fix worked when PHP is still enforcing a lower limit.
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Where to put custom definitions
Open the active wp-config.php in the site’s document root (or the location used by your host). Add custom define() statements above the editing marker:
/* Custom settings go above this line. */
define( 'WP_DEBUG', true );
/* That's all, stop editing! Happy blogging. */Back up the file before editing, preserve its PHP syntax, and test the site immediately after saving. A missing semicolon or an unmatched quote can produce a “critical error” before WordPress can render its own recovery screen.
URL constants: WP_HOME versus WP_SITEURL
These two constants solve different URL problems:
| Constant | Controls | Typical value | Dashboard effect |
|---|---|---|---|
WP_HOME |
The public address visitors use for the site | https://example.com |
The corresponding General Settings field is shown but cannot be edited |
WP_SITEURL |
The address where the WordPress application files are located | https://example.com/wp |
The WordPress Address field is shown but cannot be edited |
WordPress uses these values at runtime instead of the matching values stored in the wp_options table. Defining a constant does not rewrite those database rows. Remove the definitions later and the database values become relevant again.
Subdirectory installation example
If WordPress core is installed in /wp while the public site remains at the domain root, use:
define( 'WP_HOME', 'https://example.com' );
define( 'WP_SITEURL', 'https://example.com/wp' );This arrangement is useful after a migration, behind a reverse proxy, or whenever the public URL and application URL differ. Both values must use the correct scheme (http versus https), hostname, port and path. A typo can send visitors between two URLs indefinitely, generate mixed-content warnings, or point CSS and JavaScript requests at the wrong directory.
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Recovering from a bad URL
- Use file manager, SFTP or SSH to open
wp-config.php. - Check for duplicate
WP_HOMEorWP_SITEURLdefinitions; the first effective definition should be the intended one. - Set both values to the exact canonical URLs, including any subdirectory and without a trailing slash.
- Clear browser, proxy and page-cache entries, then test the front end and
/wp-admin/. - Once the site is stable, decide whether the constants should remain as deliberate deployment settings or be removed so the dashboard/database values control the URLs.
Debugging constants and safe error visibility
WP_DEBUG enables WordPress debug mode. It is intended primarily for development and staging copies, where notices, deprecated calls and warnings can be investigated without exposing them to visitors.
Logging and displaying errors are separate controls. A practical development or staging block is:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', false );
@ini_set( 'display_errors', 0 );
define( 'SCRIPT_DEBUG', true );What each setting does
| Constant | Purpose | Recommended handling |
|---|---|---|
WP_DEBUG |
Turns on WordPress debug mode | Use on local or staging environments while reproducing a fault |
WP_DEBUG_LOG |
Writes errors to wp-content/debug.log |
Keep enabled while collecting a reproducible error; protect the log from public access |
WP_DEBUG_DISPLAY |
Controls whether notices and warnings appear in generated HTML | Set to false on any public site |
SCRIPT_DEBUG |
Loads non-minified WordPress JavaScript and CSS | Enable when tracing an asset or JavaScript problem; disable for normal production operation |
A disciplined debugging workflow
- Take a backup and reproduce the problem on staging whenever possible.
- Place the definitions above the editing marker and load the failing page or administrator action once.
- Read
wp-content/debug.log, noting the first relevant error, file path, plugin or theme name, and line number. - Disable the suspected extension or roll back the offending change, then reproduce the request to confirm the cause.
- After troubleshooting, set
WP_DEBUG,WP_DEBUG_LOGandSCRIPT_DEBUGtofalse(or remove the temporary definitions) and keepWP_DEBUG_DISPLAYdisabled.
Never leave full error output enabled on a public production site. Stack traces can reveal filesystem paths, SQL details, usernames and other information useful to an attacker.
Memory limits: WP_MEMORY_LIMIT and WP_MAX_MEMORY_LIMIT
WP_MEMORY_LIMIT is the amount of PHP memory WordPress requests for front-end requests. WP_MAX_MEMORY_LIMIT is used for administration and heavier operations such as updates, imports and image processing.
The WordPress Hosting Handbook documents these defaults:
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| Constant | Scope | Documented default |
|---|---|---|
WP_MEMORY_LIMIT |
Single-site front end | 40M |
WP_MEMORY_LIMIT |
Multisite front end | 64M |
WP_MAX_MEMORY_LIMIT |
Administration and intensive tasks | 256M when PHP permits a runtime increase and the existing limit is not already higher or unlimited |
These are documented WordPress defaults, not a guarantee from your host. To request higher values, add for example:
define( 'WP_MEMORY_LIMIT', '128M' );
define( 'WP_MAX_MEMORY_LIMIT', '256M' );Why increasing the constant may not fix a fatal error
WordPress can request a PHP limit, but it cannot defeat a server configuration that blocks ini_set() or enforces a lower php.ini memory_limit. In that case, the host-level setting remains the effective ceiling. A hosting control panel, PHP information page or support response can confirm the actual limit.
More memory can also conceal the real defect. A runaway query, inefficient plugin, oversized import, corrupted image or theme loop may eventually exhaust any allowance. If the fatal error persists after the effective PHP limit is raised, inspect the log, identify the operation consuming memory, test with plugins and the theme isolated, and reduce the import or image workload before requesting still more memory.
What WP_CACHE really does
WP_CACHE is a bootstrap signal, not a cache engine. When it is true, WordPress attempts to load wp-content/advanced-cache.php early in the request. Defining the constant alone does not create cached pages, install a plugin or configure cache rules.
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If advanced-cache.php is absent, the constant has little or no practical effect. Page-cache systems may create that drop-in and add their own settings. Persistent object caching is a separate layer: an object-cache.php drop-in can connect WordPress to a Redis- or Memcached-style service without relying on WP_CACHE.
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| Layer | Drop-in | Role |
|---|---|---|
| Page-cache bootstrap | advanced-cache.php |
Loads early when WP_CACHE is true |
| Persistent object cache | object-cache.php |
Stores database/query objects between requests; separate from WP_CACHE |
When disabling a page-cache system, remove or deactivate its drop-in according to that system’s instructions rather than assuming that changing WP_CACHE alone clears cached files.
Security salts, keys and database settings
The sample configuration includes authentication keys and salts generated by WordPress.org’s secret-key service. They make login cookies and related tokens unpredictable. Treat them, database credentials and any API secrets in the file as confidential: keep the file out of public repositories, restrict filesystem permissions and avoid pasting its contents into support forums.
If salts may have been exposed, rotate them. WP-CLI provides the documented command:
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Filesystem and installation constants
WP_CONTENT_DIR
WP_CONTENT_DIR is the absolute filesystem path to the wp-content directory. It is useful when content lives outside the default location, but the value must be a server filesystem path, not a browser URL. A mismatched path can prevent WordPress from finding plugins, themes, uploads or drop-ins.
Database constants and table prefix
The same file normally defines DB_NAME, DB_USER, DB_PASSWORD, DB_HOST and $table_prefix. These values determine which database WordPress opens and which table names it queries. Changing them without migrating the database can make a healthy installation appear empty or produce connection errors.
Choosing settings by environment
| Setting area | Local development | Staging | Public production |
|---|---|---|---|
| Debug mode | WP_DEBUG true |
Usually true while testing | Normally false after diagnosis |
| Error display | Optional for a private machine | Prefer false and inspect logs | False |
| Script debug | Enable when working on assets | Enable for reproduction | False unless actively diagnosing |
| URL constants | Match the local domain | Match the staging domain | Use only when deliberately overriding database values |
| Memory values | Enough for the tools being tested | Match production limits where possible | Raise only after checking the host ceiling and root cause |
Common failure modes and fixes
The dashboard URL fields are locked
Check whether WP_HOME or WP_SITEURL is defined. Those constants intentionally override the database values and prevent editing the corresponding fields. Change the file, not the form.
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Compare both URL constants with the canonical domain, HTTPS status, port and subdirectory. Remove stale definitions, clear proxy and page caches, and verify that the reverse proxy passes the correct scheme.
Visitors see PHP warnings
Set WP_DEBUG_DISPLAY to false and disable PHP display errors, while leaving logging enabled long enough to capture the fault. Do not treat hiding output as fixing the underlying error.
A cache constant appears to do nothing
Inspect whether wp-content/advanced-cache.php exists and whether the page-cache system that created it is active. For object caching, check separately for object-cache.php and its service connection.
WordPress still reports an exhausted memory limit
Confirm the effective PHP memory_limit, not merely the requested WordPress value. Then investigate the plugin, theme, import or image-processing task that consumes memory.
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All users are logged out unexpectedly
Authentication salts were likely changed. This is expected after a deliberate salt rotation; verify the change was authorized and have users sign in again.
A safe editing checklist
- Back up
wp-config.phpand the database. - Use the active file for the correct environment; managed hosts sometimes keep configuration outside the web root.
- Place definitions above the editing marker.
- Use single quotes, semicolons and valid PHP syntax.
- Define each constant once and remove stale duplicate blocks.
- Keep secrets out of version-control logs and public bug reports.
- Test the front end, login, media upload, updates and one representative administrator task.
- Disable temporary debugging and script settings after diagnosis.
Handled this way, wp-config.php remains a precise control layer: URL constants separate the public and application addresses, debug constants separate logging from display, memory constants express a request rather than override the server, and cache behavior depends on the correct drop-in rather than on a single switch.
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