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Skill 82 · Instrument Product Analytics
Subchapter 82.48
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This is an optional library you can install if you’re working with PHP. It uses an internal queue to batch requests, flushes at the end of the request, and optionally does so in an async manner.
Install the package with Composer:
Terminal
composer require posthog/posthog-phpIn your app, set your project token before making any calls.
PHP
PostHog\PostHog::init("<ph_project_token>",
['host' => 'https://us.i.posthog.com']
);Note: As a rule of thumb, we do not recommend having API keys or tokens in plaintext. Setting them as environment variables is best. The PHP SDK reads
POSTHOG_API_KEYandPOSTHOG_HOSTwhen you omit the project token or host.
You can find your project token and instance address in the project settings (opens in a new tab) page in PostHog.
Identifying users is required. Backend events need a
distinct_idthat matches the ID your frontend uses when callingposthog.identify(). Without this, backend events are orphaned — they can’t be linked to frontend event captures, session replays (opens in a new tab), LLM traces (opens in a new tab), or error tracking (opens in a new tab).See our guide on identifying users (opens in a new tab) for how to set this up.
You can send custom events using capture:
PHP
PostHog::capture([
'distinctId' => 'distinct_id_of_the_user',
'event' => 'user_signed_up'
]);Tip: We recommend using a
[object] [verb]format for your event names, where[object]is the entity that the behavior relates to, and[verb]is the behavior itself. For example,project created,user signed up, orinvite sent.
Optionally, you can include additional information with the event by including a properties (opens in a new tab) object:
PHP
PostHog::capture([
'distinctId' => 'distinct_id_of_the_user',
'event' => 'user_signed_up',
'properties' => [
'login_type' => 'email',
'is_free_trial' => 'true'
]
]);If you’re aiming for a backend-only implementation of PostHog and won’t be capturing events from your frontend, you can send pageviews from your backend like so:
PHP
PostHog::capture([
'distinctId' => 'distinct_id_of_the_user',
'event' => '$pageview',
'properties' => [
'$current_url' => 'https://example.com'
]
]);The PHP SDK captures identified events by default. These create person profiles (opens in a new tab). To set person properties (opens in a new tab), call identify with the user’s distinct ID and properties:
PHP
PostHog::identify([
'distinctId' => 'distinct_id',
'properties' => [
'email' => 'max@example.com',
'name' => 'Max Hedgehog',
],
]);You can also include person properties when capturing an event:
PHP
PostHog::capture([
'distinctId' => 'distinct_id',
'event' => 'event_name',
'properties' => [
'$set' => [
'name' => 'Max Hedgehog'
],
'$set_once' => [
'initial_url' => '/blog'
]
]
]);For more details on the difference between $set and $set_once, see our person properties docs (opens in a new tab).
To capture anonymous events (opens in a new tab) without person profiles, set the event’s $process_person_profile property to false:
PHP
PostHog::capture([
'distinctId' => 'distinct_id',
'event' => 'event_name',
'properties' => [
'$process_person_profile' => false
]
]);Sometimes, you want to assign multiple distinct IDs to a single user. This is helpful when your primary distinct ID is inaccessible. For example, if a distinct ID used on the frontend is not available in your backend.
In this case, you can use alias to assign another distinct ID to the same user.
PHP
PostHog::alias([
'distinctId' => 'distinct_id',
'alias' => 'alias_id'
]);We strongly recommend reading our docs on alias (opens in a new tab) to best understand how to correctly use this method.
PostHog’s feature flags (opens in a new tab) enable you to safely deploy and roll back new features as well as target specific users and groups with them.
There are two steps to implement feature flags in PHP:
Call PostHog::evaluateFlags() once for the user, then read values from the returned snapshot.
PHP
$flags = PostHog::evaluateFlags('distinct_id_of_your_user');
if ($flags->isEnabled('flag-key')) {
// Do something differently for this user
// Optional: fetch the payload
$matchedFlagPayload = $flags->getFlagPayload('flag-key');
}PHP
$flags = PostHog::evaluateFlags('distinct_id_of_your_user');
$enabledVariant = $flags->getFlag('flag-key');
if ($enabledVariant === 'variant-key') { // replace 'variant-key' with the key of your variant
// Do something differently for this user
// Optional: fetch the payload
$matchedFlagPayload = $flags->getFlagPayload('flag-key');
}$flags->getFlag() returns the variant string for multivariate flags, true for enabled boolean flags, false for disabled flags, and null when the flag wasn’t returned by the evaluation.
You can also call $flags->getKeys() to list the evaluated flag keys, or $flags->getEventProperties() to get the $feature/<flag-key> and $active_feature_flags properties that would be attached to a captured event.
Note:
PostHog::isFeatureEnabled(),PostHog::getFeatureFlag(),PostHog::getFeatureFlagPayload(), andcapture(['send_feature_flags' => true])still work during the migration period, but they’re deprecated. PreferevaluateFlags()for new code.
If you want use your feature flag to breakdown or filter events in your insights (opens in a new tab), you’ll need to include feature flag information in those events. This ensures that the feature flag value is attributed correctly to the event.
Note: This step is only required for events captured using our server-side SDKs or API (opens in a new tab).
There are two methods you can use to include feature flag information in your events:
Pass the same flags object that you used for branching. This attaches the exact flag values from that evaluation and doesn’t make another /flags request.
PHP
$flags = PostHog::evaluateFlags('distinct_id_of_your_user');
if ($flags->isEnabled('flag-key')) {
// Do something differently for this user
}
PostHog::capture([
'distinctId' => 'distinct_id_of_your_user',
'event' => 'event_name',
'flags' => $flags,
]);By default, this attaches every flag in the snapshot using $feature/<flag-key> properties and $active_feature_flags.
To reduce event property bloat, pass a filtered snapshot:
PHP
// Attach only flags accessed with isEnabled() or getFlag() before this call
PostHog::capture([
'distinctId' => 'distinct_id_of_your_user',
'event' => 'event_name',
'flags' => $flags->onlyAccessed(),
]);
// Attach only specific flags
PostHog::capture([
'distinctId' => 'distinct_id_of_your_user',
'event' => 'event_name',
'flags' => $flags->only(['checkout-flow', 'new-dashboard']),
]);onlyAccessed() is order-dependent. If you call it before accessing any flags with isEnabled() or getFlag(), no feature flag properties are attached.
In the event properties, include $feature/feature_flag_name: variant_key:
PHP
PostHog::capture([
'distinctId' => 'distinct_id_of_your_user',
'event' => 'event_name',
'properties' => [
// Replace feature-flag-key with your flag key and 'variant-key' with the key of your variant
'$feature/feature-flag-key' => 'variant-key',
],
]);By default, evaluateFlags() evaluates every flag for the user. If you only need a few flags, pass flagKeys to request only those flags:
PHP
$flags = PostHog::evaluateFlags(
distinctId: 'distinct_id_of_your_user',
flagKeys: ['checkout-flow', 'new-dashboard'],
);evaluateFlags() also accepts optional parameters for local evaluation and GeoIP behavior:
PHP
$flags = PostHog::evaluateFlags(
distinctId: 'distinct_id_of_your_user',
groups: ['company' => 'company_id_in_your_db'],
personProperties: ['plan' => 'pro'],
groupProperties: ['company' => ['employees' => 11]],
onlyEvaluateLocally: false, // Defaults to false. Set to true to avoid a remote fallback.
disableGeoip: false, // Defaults to false. Set to true to disable GeoIP enrichment during remote evaluation.
flagKeys: ['checkout-flow', 'new-dashboard'],
);Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights (opens in a new tab) on the flag. With evaluateFlags(), the SDK sends this event when you call $flags->isEnabled() or $flags->getFlag() for a flag.
The SDK deduplicates these events per (flag key, distinct_id) in a local cache. If you reinitialize the PostHog client, the cache resets and $feature_flag_called events may be sent again. PostHog handles duplicates, so duplicate $feature_flag_called events don’t affect your analytics.
$flags->getFlagPayload() doesn’t send $feature_flag_called events and doesn’t count as an access for onlyAccessed().
Sometimes, you may want to evaluate feature flags using person properties (opens in a new tab), groups (opens in a new tab), or group properties that haven’t been ingested yet, or were set incorrectly earlier.
You can provide properties to evaluate the flag with by using the person properties, groups, and group properties arguments. PostHog will then use these values to evaluate the flag, instead of any properties currently stored on your PostHog server.
For example:
PHP
$flags = PostHog::evaluateFlags(
distinctId: 'distinct_id_of_the_user',
groups: [
'your_group_type' => 'your_group_id',
'another_group_type' => 'your_group_id',
],
personProperties: ['property_name' => 'value'],
groupProperties: [
'your_group_type' => ['group_property_name' => 'value'],
'another_group_type' => ['group_property_name' => 'value'],
],
);
if ($flags->isEnabled('flag-key')) {
// Do something differently for this user
}By default, a user’s GeoIP properties are set using the IP address they use to capture events on the frontend. You may want to override the these properties when evaluating feature flags. A common reason to do this is when you’re not using PostHog on your frontend, so the user has no GeoIP properties.
You can override GeoIP properties by including them in the person_properties parameter when evaluating feature flags. This is useful when you’re evaluating flags on your backend and want to use the client’s location instead of your server’s location.
The following GeoIP properties can be overridden:
$geoip_country_code$geoip_country_name$geoip_city_name$geoip_city_confidence$geoip_continent_code$geoip_continent_name$geoip_latitude$geoip_longitude$geoip_postal_code$geoip_subdivision_1_code$geoip_subdivision_1_name$geoip_subdivision_2_code$geoip_subdivision_2_name$geoip_subdivision_3_code$geoip_subdivision_3_name$geoip_time_zoneSimply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.
You can configure the feature_flag_request_timeout_ms parameter when initializing your PostHog client to set a flag request timeout. This helps prevent your code from being blocked if PostHog’s servers are too slow to respond. By default, this is set to 3 seconds.
PHP
PostHog::init("<ph_project_token>",
[
'host' => 'https://us.i.posthog.com',
'feature_flag_request_timeout_ms' => 3000, // Time in milliseconds. Defaults to 3000 (3 seconds).
]
);Evaluating feature flags requires making a request to PostHog for each flag. However, you can improve performance by evaluating flags locally. Instead of making a request for each flag, PostHog will periodically request and store feature flag definitions locally, enabling you to evaluate flags without making additional requests.
It is best practice to use local evaluation flags when possible, since this enables you to resolve flags faster and with fewer API calls.
To load feature flag definitions for local evaluation, initialize the SDK with your feature flags secure API key as personalAPIKey:
PHP
PostHog::init(
'<ph_project_token>',
['host' => 'https://us.i.posthog.com'],
personalAPIKey: 'your feature flags secure API key'
);For details on how to implement local evaluation, see our local evaluation guide (opens in a new tab). For distributed or stateless PHP applications, use flag_definition_cache_provider to share flag definitions across workers or requests. See local evaluation in distributed environments (opens in a new tab).
Since experiments (opens in a new tab) use feature flags, the code for running an experiment is very similar to the feature flags code:
PHP
$flags = PostHog::evaluateFlags('user_distinct_id');
$variant = $flags->getFlag('experiment-feature-flag-key');
if ($variant === 'variant-name') {
// Do something differently for this user
}It’s also possible to run experiments without using feature flags (opens in a new tab).
Group analytics allows you to associate an event with a group (e.g. teams, organizations, etc.). This feature requires version 2.1.0 or above of the PHP SDK. Read the group analytics guide (opens in a new tab) for more information.
Note: This is a paid feature and is not available on the open-source or free cloud plan. Learn more on the pricing page (opens in a new tab).
To create a group or update its properties, use groupIdentify:
PHP
PostHog::groupIdentify([
'groupType' => 'company',
'groupKey' => 'company_id_in_your_db',
'properties' => [
'name' => 'Awesome Inc.',
'employees' => 11,
],
// Optional distinct ID to associate this event with an existing person.
// Requires posthog-php 4.4.0 or later.
'distinctId' => 'user_distinct_id'
]);name is a special property which is used in the PostHog UI for the name of the group. If you don’t specify a name property, the group ID is used instead.
If the optional distinctId parameter is not provided in the group identify call, it defaults to ${groupType}_${groupKey} (e.g., $company_company_id_in_your_db in the example above). This default behavior results in each group appearing as a separate person in PostHog. To avoid this, use a consistent distinctId, such as group_identifier, or a real user distinct ID.
Once a group is created, you can use the capture method and pass in the groups parameter to capture an event with group analytics.
PHP
PostHog::capture([
'distinctId' => 'user_distinct_id',
'event' => 'some_event',
'groups' => ['company' => 'company_id_in_your_db']
]);Use request context to apply a distinct ID, session ID, and common properties to all captures inside a callback. This is useful when connecting frontend activity to backend events, session replay, and error tracking.
PHP
PostHog::withContext([
'distinctId' => 'user_distinct_id',
'sessionId' => 'session_id_from_frontend',
'properties' => [
'$current_url' => 'https://example.com/account',
],
], function () {
PostHog::capture([
'event' => 'backend_event',
]);
});You can extract PostHog context from frontend tracing headers with contextFromHeaders(). If you’re using PostHog JS (opens in a new tab) on the frontend, configure tracing_headers (opens in a new tab) for your PHP backend hostname so browser requests include the session and distinct ID headers.
Then read the incoming headers on the server:
PHP
$context = PostHog::contextFromHeaders($_SERVER);
PostHog::withContext($context, function () {
PostHog::capture([
'event' => 'backend_event',
]);
});Call PostHog::getContext() to read the currently active context. Pass ['fresh' => true] as the third argument to withContext() if you don’t want to inherit any existing context.
Tracing headers are client-controlled analytics context, not authentication or authorization. Pass an authenticated distinctId explicitly for security-sensitive server-side decisions.
The PHP SDK supports both manual exception capture and opt-in automatic error tracking.
To automatically capture uncaught exceptions, PHP errors, and fatal shutdown errors, enable error_tracking when initializing the client:
PHP
PostHog::init(
'<ph_project_token>',
[
'host' => 'https://us.i.posthog.com',
'error_tracking' => [
'enabled' => true,
],
],
);You can also call PostHog::captureException() directly for manual capture. When source files are readable at runtime, PostHog includes surrounding source lines for in-app stack frames automatically.
For the full setup guide, including context_provider, excluded exceptions, and verification steps, see the PHP error tracking installation docs (opens in a new tab).
When calling PostHog::init, there are various configuration options you can set apart from the host. Pass them into your client initialisation like so:
PHP
PostHog::init(
'<ph_project_token>',
[
'host' => 'https://us.i.posthog.com',
'debug' => true,
'ssl' => false,
// all options go here
],
);All possible options below:
| Attribute | Description |
|---|---|
host Type: String Default: us.i.posthog.com | URL of your PostHog instance. |
ssl Type: Boolean Default: true | Whether to use SSL for API requests or not. If host includes http:// or https://, the SDK infers this option unless you set it explicitly. |
timeout Type: Integer Default: 10000 | Request timeout in milliseconds. |
verify_batch_events_request Type: Boolean Default: true | Whether to verify successful delivery of batch events (true, synchronous) or fire and forget (false, asynchronous) with the lib_curl consumer. |
feature_flag_request_timeout_ms Type: Integer Default: 3000 | Request timeout for feature flags in milliseconds. |
flag_definition_cache_provider Type: PostHog\FlagDefinitionCacheProvider Default: null | Provider for distributed local-evaluation flag definition caching. See local evaluation in distributed environments (opens in a new tab). |
maximum_backoff_duration Type: Integer Default: 10000 | Request retry backoff. Retries stop after this duration is hit. |
consumer Type: String Default: lib_curl | One of socket, file, lib_curl, fork_curl, and noop. Determines what transport option to use for analytics capture. |
debug Type: Boolean Default: false | Output debug logs or not. |
max_queue_size Type: Integer Default: 1000 | Maximum number of events to queue before rejecting new events. Applies to queued consumers. |
batch_size Type: Integer Default: 100 | Number of queued events to send in each batch. Applies to queued consumers. |
compress_request Type: Boolean/String Default: false | Whether to gzip batch request payloads. |
error_handler Type: Callable Default: null | Callback invoked for SDK transport errors. |
filename Type: String Default: sys_get_temp_dir() . '/posthog.log' | File path used when consumer is set to file. |
error_tracking Type: Array Default: [] | Enables automatic error tracking. See the options below or the PHP error tracking setup guide (opens in a new tab). |
| Attribute | Description |
|---|---|
enabled Type: Boolean Default: false | Enables automatic error tracking handlers. Manual captureException works regardless. |
capture_errors Type: Boolean Default: true | When enabled, captures PHP errors and fatal shutdown errors in addition to uncaught exceptions. |
excluded_exceptions Type: Array of class strings Default: [] | Throwable classes to skip during automatic capture. |
max_frames Type: Integer Default: 20 | Maximum number of stack frames included in $exception_list. |
context_provider Type: Callable or null Default: null | Callback that returns distinctId and extra event properties for automatic captures. |
Call PostHog::flush() to send queued events without closing resources. When a script or long-running worker stops, call PostHog::shutdown() instead; it flushes queued events and releases resources held by providers such as flag_definition_cache_provider.
PHP
PostHog::shutdown();PHP
PostHog::init(
'<ph_project_token>',
[
'host' => 'https://us.i.posthog.com',
'debug' => true,
],
);This library is largely based on the analytics-php package.
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