Instrument Integration

AI agents: this is one page from PostHog's docs. Full index of Markdown docs for LLMs: https://posthog.com/llms.txt

PHP

Canonical URL: https://posthog.com/docs/libraries/php

Cloud region: This page uses US Cloud hosts. For an EU Cloud project, use https://eu.i.posthog.com instead of https://us.i.posthog.com (ingestion host), and use https://eu.posthog.com instead of https://app.posthog.com (app host for links).

On this page

• Installation

• Identifying users

• Capturing events

• Person profiles and properties

• Alias

• Feature flags

• Request context

• Error tracking

• Config options

• Flushing and shutting down

• Debug mode

• Thank you

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.

Using Laravel? See our Laravel guide for the experimental Laravel SDK or manual integration with the PHP SDK. Using the PHP SDK directly with your own instrumentation remains supported.

Installation

Install the package with Composer:

composer require posthog/posthog-php

In your app, set your project token before making any calls.

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_KEY and POSTHOG_HOST when you omit the project token or host.

You can find your project token and instance address in the project settings page in PostHog.

Identifying users

Identifying users is required. Backend events need a distinct_id that matches the ID your frontend uses when calling posthog.identify(). Without this, backend events are orphaned — they can't be linked to frontend event captures, session replays, LLM traces, or error tracking.

See our guide on identifying users for how to set this up.

Capturing events

You can send custom events using capture:

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, or invite sent.

Setting event properties

Optionally, you can include additional information with the event by including a properties object:

PostHog::capture([
  'distinctId' => 'distinct_id_of_the_user',
  'event' => 'user_signed_up',
  'properties' => [
    'login_type' => 'email',
    'is_free_trial' => 'true'
  ]
]);

Sending page views

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:

PostHog::capture([
  'distinctId' => 'distinct_id_of_the_user',
  'event' => '$pageview',
  'properties' => [
    '$current_url' => 'https://example.com'
  ]
]);

Person profiles and properties

The PHP SDK captures identified events by default. These create person profiles. To set person properties, call identify with the user's distinct ID and properties:

PostHog::identify([
    'distinctId' => 'distinct_id',
    'properties' => [
        'email' => '[email protected]',
        'name' => 'Max Hedgehog',
    ],
]);

You can also include person properties when capturing an event:

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.

To capture anonymous events without person profiles, set the event's $process_person_profile property to false:

PostHog::capture([
    'distinctId' => 'distinct_id',
    'event' => 'event_name',
    'properties' => [
        '$process_person_profile' => false
    ]
]);

Alias

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.

PostHog::alias([
  'distinctId' => 'distinct_id',
  'alias' => 'alias_id'
]);

We strongly recommend reading our docs on alias to best understand how to correctly use this method.

Feature flags

PostHog's feature flags 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:

Step 1: Evaluate flags once

Call PostHog::evaluateFlags() once for the user, then read values from the returned snapshot.

Boolean feature flags

$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');
}

Multivariate feature flags

$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(), and capture(['send_feature_flags' => true]) still work during the migration period, but they're deprecated. Prefer evaluateFlags() for new code.

Step 2: Include feature flag information when capturing events

If you want use your feature flag to breakdown or filter events in your insights, 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.

There are two methods you can use to include feature flag information in your events:

Method 1: Pass the evaluated flags snapshot to capture()

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.

$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:

// 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.

Method 2: Include the $feature/feature_flag_name property manually

In the event properties, include $feature/feature_flag_name: variant_key:

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',
    ],
]);

Evaluating only specific flags

By default, evaluateFlags() evaluates every flag for the user. If you only need a few flags, pass flagKeys to request only those flags:

$flags = PostHog::evaluateFlags(
    distinctId: 'distinct_id_of_your_user',
    flagKeys: ['checkout-flow', 'new-dashboard'],
);

Optional evaluation parameters

evaluateFlags() also accepts optional parameters for local evaluation and GeoIP behavior:

$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'],
);

Sending $feature_flag_called events

Capturing $feature_flag_called events enables PostHog to know when a flag was accessed by a user and provide analytics and insights 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().

Advanced: Overriding server properties

Sometimes, you may want to evaluate feature flags using person properties, groups, 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:

$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
}

Overriding GeoIP properties

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_zone

Simply include any of these properties in the person_properties parameter alongside your other person properties when calling feature flags.

Request timeout

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.

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).
    ]
);

Local Evaluation

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 secret API key as personalAPIKey:

PostHog::init(
    '<ph_project_token>',
    ['host' => 'https://us.i.posthog.com'],
    personalAPIKey: 'your secret API key'
);

For details on how to implement local evaluation, see our local evaluation guide. 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.

Experiments (A/B tests)

Since experiments use feature flags, the code for running an experiment is very similar to the feature flags code:

$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.

Group analytics

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 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.

To create a group or update its properties, use groupIdentify:

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.

PostHog::capture([
    'distinctId' => 'user_distinct_id',
    'event' => 'some_event',
    'groups' => ['company' => 'company_id_in_your_db']
]);

Request context

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.

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 on the frontend, configure tracing_headers for your PHP backend hostname so browser requests include the session and distinct ID headers.

Then read the incoming headers on the server:

$context = PostHog::contextFromHeaders($_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.

Error tracking

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:

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.

Config options

When calling PostHog::init, there are various configuration options you can set apart from the host. Pass them into your client initialisation like so:

PostHog::init(
    '<ph_project_token>',
    [
        'host' => 'https://us.i.posthog.com',
        'debug' => true,
        'ssl' => false,
        // all options go here
    ],
);

All possible options below:

Error tracking options

Flushing and shutting down

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.

PostHog::shutdown();

Debug mode

PostHog::init(
    '<ph_project_token>',
    [
        'host' => 'https://us.i.posthog.com',
        'debug' => true,
    ],
);

Thank you

This library is largely based on the analytics-php package.