## What happened
Copy-pasting the PHP send-SMS example from Infobip's interactive IDE (tryapi.infobip.com) into a project on infobip-api-php-client 5.0 crashed with an undefined method call: ObjectSerializer::sanitizeForSerialization does not exist (removed in 5.0), and further down the same class the config getters like getApiKeyWithPrefix were also gone. PHP 8.2, Symfony 6.2. The IDE examples were stale, written against the v4 API surface.
## The fix that worked
The interactive-IDE examples were generated against v4 while the 5.0 library had removed those classes and methods. Use the current SmsApi class from the v5 README instead of the obsolete IDE-generated classes (SendSmsApi etc.), and follow the README examples rather than the IDE snippets. The obsolete API classes were fully removed in release 5.1.0, so upgrade to 5.1.0+ and use SmsApi.