A blank response from curl is a symptom, not a diagnosis. Start by inspecting the complete HTTP exchange with curl -i -v https://example.test/file.php, then repeat the request while watching Nginx and PHP-FPM logs. The most common causes are an incorrect SCRIPT_FILENAME, a PHP-FPM socket that is unavailable or points to the wrong pool, a missing or unreadable script, or a PHP fatal error hidden because display_errors is disabled.
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What a blank curl response actually tells you
Seeing no HTML in a terminal does not prove that Nginx returned an empty successful page. The body may be empty because the server sent a redirect, an error response, a zero-length 200 OK, or a PHP process that failed before producing output. It can also be a client-side misunderstanding: a redirect may point to another URL, or the requested virtual host may not be the site you intended.
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Separate the layers before changing configuration:
- HTTP layer: status code, redirects, headers, and transfer details.
- Nginx routing layer: server block,
locationmatching,root,index, andtry_files. - Filesystem layer: the script path calculated from the request and whether the worker can traverse and read it.
- FastCGI transport: the
fastcgi_passsocket or TCP endpoint and its permissions. - PHP application layer: bootstrap code, configuration, dependencies, and runtime errors.
Each layer leaves different evidence. Do not “fix” PHP code when Nginx never found the file, and do not change try_files when PHP-FPM is refusing connections.
Step 1: Capture status, headers, redirects, and the body
Use a verbose request that includes response headers and the response body:
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curl -i -v https://example.test/file.phpFor a local virtual host, send the expected host name explicitly:
curl -i -v -H 'Host: example.test' http://127.0.0.1/file.phpRead the result in this order:
- Confirm that the connection reached the intended address and protocol.
- Record the HTTP status. A
200, redirect, client error, server error, and transport failure each require a different branch. - Check for a
Locationheader. A301,302, or another 3xx response may explain why the first body appears empty. Follow redirects only after recording the original response; add-Lwhen you want to test the destination as well. - Look for
Content-Length: 0or transfer headers indicating that the server intentionally sent no body. - Check the content type. A PHP response normally has an appropriate text or HTML type; a downloaded source file means the PHP location was not selected correctly and is a security problem.
curl -I performs a headers-only probe and can be useful for a quick check, but -i -v is better for a blank-body investigation because it shows the request, response headers, redirects, and any returned body together.
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| Observed response | What it narrows down | Next evidence to collect |
|---|---|---|
200 OK with zero-length body |
PHP may have produced no output, terminated during startup, or been stopped by an application error. | PHP-FPM and application logs, followed by a direct diagnostic script. |
301 or 302 |
HTTP or application routing is sending the request elsewhere. | The Location value, host and scheme settings, and a second request to the destination. |
404 |
Nginx routing, URI mapping, or file existence is wrong before PHP runs. | Active location rules, root, try_files, and the requested path. |
502 Bad Gateway |
Nginx cannot complete the FastCGI connection. | fastcgi_pass, PHP-FPM service state, socket permissions, and timeout messages. |
| PHP source displayed or downloaded | The request was handled as static content instead of being passed to PHP-FPM. | Which location matched and whether it contains fastcgi_pass. |
Step 2: Watch Nginx and PHP-FPM logs during one request
Open separate terminals, start watching the configured Nginx error and access logs plus the PHP-FPM or application error log, and then make exactly one curl request. Correlating one request with one set of log lines is more reliable than making many configuration changes and trying to infer which one mattered.
Nginx identifies access.log and error.log as its primary troubleshooting files. Their locations depend on the distribution, virtual-host configuration, container image, and service unit. Find the active paths from the loaded configuration instead of assuming that every system uses the same filename or directory. A typical diagnostic pattern is:
sudo tail -f /path/to/nginx/error.log /path/to/nginx/access.logUse the corresponding configured PHP-FPM or application log in another terminal. The important part is not the exact path but that you are reading the log used by the running pool and application.
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Messages have distinct meanings:
- “Primary script unknown” or “No input file specified”: PHP-FPM received a script path that does not exist in its filesystem view. Check
rootandSCRIPT_FILENAME. - Permission denied: the worker cannot traverse a parent directory, read the script, or access the socket.
- Upstream connection refused, unavailable, or timed out: Nginx cannot reach the configured PHP-FPM endpoint, or the pool is overloaded or stopped.
- Application fatal error: PHP started but failed during bootstrap, configuration loading, dependency loading, or request handling.
- No PHP-FPM entry at all: the request may have matched another Nginx location, been rejected before FastCGI, or gone to a different host or container.
Keep the logs attached while testing every later change. A successful curl response without a corresponding expected access-log entry usually means you are looking at the wrong server instance or virtual host.
Step 3: Validate the PHP location block
The PHP location must map the request URI to a real script and pass it to the intended PHP-FPM endpoint. A minimal pattern is:
location ~ .php$ {
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_pass unix:/run/php/php-fpm.sock;
}The exact directives can differ by distribution and framework, but three values must agree:
root: the directory that actually contains the requested script.fastcgi_param SCRIPT_FILENAME: the absolute path PHP-FPM should open.fastcgi_pass: the live Unix socket or TCP address for the PHP-FPM service and pool you intend to use.
Nginx documentation states: “The SCRIPT_FILENAME parameter is used in PHP for determining the script name.” With the common expression $document_root$fastcgi_script_name, a request for /file.php under a root of /var/www/example/public becomes /var/www/example/public/file.php. That computed path must be the real path from the perspective of the PHP-FPM process, not merely a path that exists inside Nginx’s container or in a different deployment release.
Inspect the loaded configuration, not only the file you edited. Includes, an earlier regular-expression location, or a different server block may override the fragment you are reading. After a change, validate the syntax and reload the service through your platform’s normal procedure. A syntax check that fails must be corrected before any reload is attempted.
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Step 4: Confirm the PHP-FPM endpoint
fastcgi_pass can target a Unix socket such as unix:/run/php/php-fpm.sock or a TCP endpoint such as 127.0.0.1:9000. The socket path and port are version-, distribution-, pool-, and container-specific; do not copy an example path without checking the running service.
- Read the active Nginx configuration and record the exact
fastcgi_passvalue. - For a Unix socket, verify that the socket exists at that exact path and that the Nginx worker can connect to it.
- For TCP, verify that PHP-FPM is listening on the configured address and port, and that network policy permits the connection.
- Check the PHP-FPM service and pool state. A stopped service, crashed worker manager, wrong PHP version, or different pool can all leave Nginx pointing at an endpoint that is present in configuration but absent at runtime.
- Read the Nginx error log while making one request. Connection-refused and timeout messages distinguish transport failure from a script-path failure.
If the endpoint belongs to a different PHP version or pool, the request may reach PHP but use a different configuration, extension set, user, or filesystem namespace. Match the socket or port to the application’s active PHP-FPM service rather than choosing the first endpoint that exists.
Step 5: Prove the calculated file path exists and is readable
Derive the final path exactly as Nginx does, then test it on the server. For /file.php, concatenate the active root and fastcgi_script_name; do not substitute a path from a development machine.
- Confirm that the file exists at that exact location.
- Check every parent directory for execute (traverse) permission.
- Check that the PHP-FPM worker user can read the file.
- Check symlink targets. A link may point outside the deployed document root or to a path that is not mounted in the PHP-FPM container.
- Check for release-directory or container mismatches, where Nginx and PHP-FPM see different filesystem roots.
- Check accidental path changes such as an extra slash, a stale deployment directory, or a document root that points at the project directory instead of its public directory.
Test permissions as the actual service account where possible. The account name varies by operating system and pool configuration, so identify it from the PHP-FPM pool rather than assuming a universal user. A file that is readable by your interactive account can still be unreadable by the worker.
Use try_files without breaking front-controller routing
Nginx can verify that a PHP file exists before passing it upstream. Its documentation describes try_files as checking the existence of the PHP file before handing the request to FastCGI. A direct-script pattern might look like:
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location ~ .php$ {
try_files $fastcgi_script_name =404;
include fastcgi_params;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_pass unix:/run/php/php-fpm.sock;
}This prevents a missing script from being sent to PHP-FPM and makes a path error an explicit 404. However, frameworks that route every friendly URL through index.php need a different arrangement: the outer application location should route the URI to the front controller, while the PHP location handles the resulting real script. Blindly appending =404 to a front-controller setup can reject valid application routes.
Use the request model that matches your application:
- Direct PHP files: check the requested script before FastCGI.
- Front controller: test the public entry script and preserve the framework’s fallback routing.
- Static-only path: do not send it to PHP merely because the URI contains a suffix that resembles a script.
Step 6: Separate PHP execution from the application
Create a temporary diagnostic file in the same document root, with deliberately simple output:
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<?php
header('Content-Type: text/plain');
echo 'php-ok';
Request that file directly with curl. Remove it immediately after testing.
- If it returns
php-ok: Nginx, the FastCGI path, and basic PHP execution are working. The original application likely fails while loading configuration, dependencies, bootstrap code, or generating output. - If it is also blank or errors: remain at the Nginx, path, permissions, socket, PHP-FPM, or logging layer. The application has not yet been isolated as the cause.
- If the source is shown: the PHP location is not being selected or does not pass the request to FastCGI. Treat this as a serious configuration issue and stop serving the file until corrected.
Use a unique filename and a controlled environment. Never leave a diagnostic script in a public production directory because it confirms execution details to anyone who can request it.
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Why PHP errors can produce an empty body
PHP’s display_errors setting controls whether errors are sent in the response. It may be disabled, especially in production, so the client can receive an empty body while the actual fatal error appears only in PHP-FPM or the application log. The PHP manual notes that changing display_errors at runtime with ini_set() does not necessarily affect fatal errors, because execution can stop before that setting takes effect.
Use the environment-appropriate approach:
- Development: temporarily enable visible errors if the environment is protected and the response cannot expose secrets. Reproduce once, capture the error, and disable verbose output afterward.
- Production: keep detailed errors out of the response, configure reliable server-side logging, and monitor the PHP-FPM and application logs. A generic HTTP error is safer than a stack trace containing credentials or filesystem paths.
- Both environments: verify that the log destination is writable, that the active PHP-FPM pool uses the configuration you changed, and that timestamps line up with the curl request.
If the logs show an application fatal error while the direct diagnostic script succeeds, fix the reported application condition rather than changing Nginx routing. Common examples include a missing dependency, invalid configuration, incompatible extension, or code path that emits no output before terminating.
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| Symptom | Likely layer | Fix to test |
|---|---|---|
200 OK with zero-length body and PHP-FPM log shows a fatal error |
PHP application | Fix the fatal error, inspect PHP-FPM and application logs, and do not rely on display_errors. |
| Nginx error log reports “Primary script unknown” or a missing input file | Path mapping or filesystem | Correct root and SCRIPT_FILENAME; verify that the computed file exists and is readable. |
502 Bad Gateway |
FastCGI transport | Check fastcgi_pass, PHP-FPM service state, socket path, permissions, and timeouts. |
404 before PHP runs |
Nginx routing | Check location matching, try_files, index, and the requested URI. |
Redirect loop or unexpected 301/302 |
HTTP or application routing | Use curl -i -v to inspect Location; verify scheme, host, and application URL settings. |
| PHP source downloaded or shown as text | Nginx handler | Ensure the PHP location is selected and uses fastcgi_pass; never serve PHP source as static content. |
Prefer reversible evidence-gathering first: a verbose curl request, a log tail, configuration inspection, and a temporary diagnostic script. Apply production behavior changes only after the failing layer is identified.
Container, deployment, and virtual-host traps
Nginx and PHP-FPM do not share the same mount
In a multi-container deployment, Nginx may see /var/www/app while PHP-FPM sees a different mount or release path. The SCRIPT_FILENAME value can look correct in Nginx logs yet point to a nonexistent path inside the PHP container. Make the application directory available at the same effective path in both containers, or use a path mapping that reflects the PHP-FPM filesystem.
The request reaches the wrong server block
A missing or unexpected Host header can select a default virtual host. Test with the intended host header, compare the access-log entry, and verify that the selected server block has the expected root and PHP location. This explains cases where editing one site’s configuration appears to have no effect.
A deployment switched the document root
Release directories and symbolic links often change during deployment. Nginx may point at the new release while PHP-FPM, a chroot, or a separate container still references the old path. Resolve the active root and symlink target at the time of the request, then test the resulting path from the PHP-FPM environment.
A repeatable decision procedure
- Run
curl -i -vand record status, headers, redirects, and body length. - Watch Nginx access and error logs plus the PHP-FPM or application log while repeating the same request.
- If the status is a redirect, inspect
Locationand correct host, scheme, or application URL routing. - If it is a 404, determine which Nginx location handled the URI and verify
root,index, andtry_files. - If it is a 502 or the error log reports an upstream failure, verify the live PHP-FPM service, socket or TCP endpoint, permissions, and timeouts.
- If the log reports an unknown script or missing input file, calculate
SCRIPT_FILENAMEand test that exact path from the worker’s filesystem and account. - If a direct diagnostic PHP file works, investigate the original application’s bootstrap and fatal errors.
- If the body is empty but logs show a fatal error, fix the application and keep production error details in server-side logs.
- Remove temporary diagnostic files and reload only the configuration that was proven to be wrong.
Production-safe checks before closing the incident
- Confirm the intended virtual host responds when addressed by its real host name.
- Confirm the PHP location passes scripts to the intended PHP-FPM pool.
- Confirm the computed script path exists, is inside the intended deployment, and is readable by the worker.
- Confirm a missing script fails predictably instead of being sent to an unrelated pool.
- Confirm application and PHP-FPM errors are recorded in monitored logs.
- Confirm verbose error display and temporary diagnostic files are disabled or removed.
- Repeat the original curl command and compare its status, headers, body, and corresponding log lines with the expected result.
A blank curl body is resolved when the evidence identifies its layer: HTTP routing, Nginx mapping, filesystem access, FastCGI transport, or PHP execution. Once those layers are checked in order, the fix is usually a precise path, endpoint, permission, routing, or application correction rather than a guess at what the browser failed to display.
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