Brevo PHP SDK v4: PHP Fatal error Class "Brevo\Client\Configuration" not found

The problem

After upgrading from getbrevo/brevo-php 2.x to 4.x, existing code fatals with: PHP Fatal error: Uncaught Error: Class "Brevo\Client\Configuration" not found. The 2.x code built a Configuration object via Brevo\Client\Configuration::getDefaultConfiguration()->setApiKey(...). That class only exists in the older SDK generation (1.x/2.x line) and the 4.x SDK does not ship it. To make it worse, the repo default branch README still showed the old Configuration-based setup at the time, which is where the confusion came from.

What fixed it

In 4.x there is no Configuration object. Initialize the unified client directly: $brevo = new Brevo('YOURAPIKEY'); then call e.g. $brevo->transactionalEmails->sendTransacEmail(...). If you cannot migrate yet, pin composer.json to the 2.x line, but the team recommends moving to the modern client. Related fix from the same thread: campaign stats per-domain rows could return null counts and crash deserialization because fields were typed non-nullable; that is fixed in 4.0.15, so pull 4.0.15+ if you use getEmailCampaigns with statsByDomain.

A user asked: After upgrading from getbrevo/brevo-php 2.x to 4.x, existing code fatals with: PHP Fatal error: Uncaught Error: Class "Brevo\Client\Configuration" not found. The 2.x code built a Configuration object via Brevo\Client\Configuration::getDefaultConfiguration()->setApiKey(...). That class only exists in the older SDK generation (1.x/2.x line) and the 4.x SDK does not ship it. To make it worse, the repo default branch README still showed the old Configuration-based setup at the time, which is where the confusion came from. The verified answer: In 4.x there is no Configuration object. Initialize the unified client directly: $brevo = new Brevo('YOURAPIKEY'); then call e.g. $brevo->transactionalEmails->sendTransacEmail(...). If you cannot migrate yet, pin composer.json to the 2.x line, but the team recommends moving to the modern client. Related fix from the same thread: campaign stats per-domain rows could return null counts and crash deserialization because fields were typed non-nullable; that is fixed in 4.0.15, so pull 4.0.15+ if you use getEmailCampaigns with statsByDomain. Thread: https://vectle.com/threads/thrah6KCObNhKHF8j2Gps6PA