PHP

This is an optional library you can install if you're working with PHP. It uses an internal queue to make calls fast and non-blocking. It also batches requests and flushes asynchronously, making it perfect to use in any part of your web app or other server-side application that needs performance.

Installation

Add the following to composer.json:

{
    "require": {
        "posthog/posthog-php": "3.0.*"
    }
}

And then install the dependencies with the command:

composer install

In your app, set your project API key before making any calls.

PostHog\PostHog::init("<ph_project_api_key>",
  array('host' => 'https://us.i.posthog.com') 
);

Note: As a rule of thumb, we do not recommend having API keys in plaintext. Setting it as an environment variable is best.

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

Capturing events

You can send custom events using capture:

PostHog::capture(array(
  '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 also include additional information in the event by setting the properties value:

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

Person profiles and properties

PostHog captures identified events by default. These create person profiles. To set person properties in these profiles, include them when capturing an event:

PostHog::capture(array(
    'distinctId' => 'distinct_id',
    'event' => 'event_name',
    'properties' => array(
        '$set' => array(
            'name' => 'Max Hedgehog'
        ),
        '$set_once' => array(
            '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(array(
    'distinctId' => 'distinct_id',
    'event' => 'event_name',
    'properties' => array(
        '$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(array(
  '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.

There are 2 steps to implement feature flags in PHP:

Step 1: Evaluate the feature flag value

Boolean feature flags

$isMyFlagEnabledForUser = PostHog::isFeatureEnabled('flag-key', 'distinct_id_of_your_user')

if ($isMyFlagEnabledForUser) {
    // Do something differently for this user
}

Multivariate feature flags

$enabledVariant = PostHog::getFeatureFlag('flag-key', 'distinct_id_of_your_user')

if ($enabledVariant === 'variant-key') { # replace 'variant-key' with the key of your variant
    # Do something differently for this user
}

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: Include the $feature/feature_flag_name property

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

PostHog::capture(array(
  'distinctId' => 'distinct_id_of_your_user',
  'event' => 'event_name',
  'properties' => array(
    '$feature/feature-flag-key'  = 'variant-key' # replace feature-flag-key with your flag key. Replace 'variant-key' with the key of your variant
  )
));

Method 2: Set send_feature_flags to true

The capture() method has an optional argument send_feature_flags, which is set to false by default. By setting this to true, feature flag information will automatically be sent with the event.

Note that by doing this, PostHog will make an additional request to fetch feature flag information before capturing the event. So this method is only recommended if you don't mind the extra API call and delay.

PostHog::capture(array(
  'distinctId' => 'distinct_id_of_your_user',
  'event' => 'event_name',
  'send_feature_flags' => true
));

Fetching all flags for a user

You can fetch all flag values for a single user by calling getAllFlags().

This is useful when you need to fetch multiple flag values and don't want to make multiple requests.

PostHog::getAllFlags('distinct_id_of_your_user')

Sending $feature_flag_called events

Capturing $feature_flag_called events enable PostHog to know when a flag was accessed by a user and thus provide analytics and insights on the flag. By default, we send a these event when:

1. You call getFeatureFlag() or isFeatureEnabled(), AND
2. It's a new user, or the value of the flag has changed.

Note: Tracking whether it's a new user or if a flag value has changed happens in a local cache. This means that if you reinitialize the PostHog client, the cache resets as well – causing $feature_flag_called events to be sent again when calling getFeatureFlag or isFeatureEnabled. PostHog is built to handle this, and so duplicate $feature_flag_called events won't affect your analytics.

You can disable automatically capturing $feature_flag_called events. For example, when you don't need the analytics, or it's being called at such a high volume that sending events slows things down.

To disable it, set the sendFeatureFlagEvents argument in your function call, like so:

$isMyFlagEnabledForUser = PostHog::isFeatureEnabled(
    key: 'flag-key',
    distinctId: 'distinct_id_of_your_user',
    sendFeatureFlagEvents: false
)

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:

PostHog::getFeatureFlag(
    'flag-key',
    'distinct_id_of_the_user',
    [
        'your_group_type' => 'your_group_id',
        'another_group_type' => 'your_group_id'
    ], // groups
    ['property_name' => 'value'], // person properties
    [
        'your_group_type' => ['group_property_name' => 'value'], 
        'another_group_type' => ['group_property_name' => 'value']
    ], // group properties
    false, // onlyEvaluateLocally, Optional. Defaults to false.
    true // sendFeatureFlagEvents
);

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.

Currently PostHog does not provide a way to override GeoIP properties using our SDKs. Our API, however, does allow you do this. See our API docs on how to override GeoIP properties for more details.

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 in the case when PostHog's servers are too slow to respond. By default, this is set at 3 seconds.

PostHog::init("<ph_project_api_key>",
  array(
    'host' => 'https://us.i.posthog.com',
    'feature_flag_request_timeout_ms' => 3000 // Time in milliseconds. Default is 3000 (3 seconds).
  ) 
);

Error handling

When using the PostHog SDK, it's important to handle potential errors that may occur during feature flag operations. Here's an example of how to wrap PostHog SDK methods in an error handler:

function handleFeatureFlag($client, $flagKey, $distinctId) {
    try {
        $isEnabled = $client->isFeatureEnabled($flagKey, $distinctId);
        echo "Feature flag '$flagKey' for user '$distinctId' is " . ($isEnabled ? 'enabled' : 'disabled') . "\n";
        return $isEnabled;
    } catch (Exception $e) {
        echo "Error fetching feature flag '$flagKey': " . $e->getMessage() . "\n";
        // Optionally, you can return a default value or throw the error
        // return false; // Default to disabled
        throw $e;
    }
}

// Usage example
try {
    $flagEnabled = handleFeatureFlag($client, 'new-feature', 'user-123');
    if ($flagEnabled) {
        // Implement new feature logic
    } else {
        // Implement old feature logic
    }
} catch (Exception $e) {
    // Handle the error at a higher level
    echo 'Feature flag check failed, using default behavior';
    // Implement fallback logic
}

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.

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

Experiments (A/B tests)

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

$variant = PostHog::getFeatureFlag('experiment-feature-flag-key', 'user_distinct_id')

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 a posthog-php version of 2.1.0 or above. 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 here.

• Capture an event and associate it with a group

PostHog::capture(array(
    'distinctId' => 'user_distinct_id',
    'event' => 'some_event',
    '$groups' => array("company" => "company_id_in_your_db")
));

• Update properties on a group

PostHog::groupIdentify(array(
    'groupType' => 'company',
    'groupKey' => 'company_id_in_your_db',
    'properties' => array("name" => "Awesome Inc.", "employees" => 11)
));

The 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 will be used instead.

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(
    'PROJECT_API_KEY',
    array(
        'host' => 'https://us.i.posthog.com', 
        'debug' => true,
        'ssl' => false,
        // all options go here
    ),
);

All possible options below:

Thank you

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