Thanks for asking question! You can inspect the log files from the browser at https://<app-name>.scm.azurewebsites.net/api/logs/docker for more details on this.
You may check if CURLOPT_SSL_VERIFYPEER option is enabled. As when making request with using cURL from PHP code regular http:// calls everything, but for https:// you need to configure few things on App Service manually for it to work correctly.
You may refer to this article: Configure a Windows PHP app for Azure App Service
Similar SO Issues below might be helpful:
https://stackoverflow.com/questions/33298712/how-to-debug-curl-in-php-and-azure
https://stackoverflow.com/questions/28419197/how-to-debug-or-fix-curl-errno-35/28420871
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