Those who are working with Paypal SDk might have come across the error Fatal error: Class 'OAuthTokenCredential' not found" This type of Class not found error occur mainly because of missing on Namespace in the file. Error generating code is given below ===================== require('vendor/autoload.php'); $sdkConfig = array( "mode" => "sandbox" ); $cred = new OAuthTokenCredential("RtUIouduouaTTdkahkVhueUEMvXO_-MCI4FGQQ4-LWvkMY5N2fGsd", "EL1tVxAjhT7cJimnz5-Nsx9k2reTKSVfErNQF-CmrwJgxRtylkGTKlU4RvrX",$sdkConfig); var_dump($cred); ===================== To fix such errors You have to specify the Namespace just before the creation of the Class object. like given below: ==================== namespace PayPal\Auth ; require('vendor/autoload.php'); $sdkConfig = array( "mode" => "sandbox" ); $cred = new OAuthTokenCredential("RtUIouduouaTTdkahkVhueUEMvXO_-MCI4FGQQ4-LW...