1 web browser use CHROME or MS EDGE, press F12 ,see CONSOLE
2 install http sniffe software FIDDLER ,
https://www.telerik.com/fiddler
3 open PHP error log in PHP.INI,
error_reporting = E_ALL
display_errors = On
log_errors = On
error_log="D:\SERVER\Logs\php56_errors.log"
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YOU will get error msg in browser console, or http request in fiddler, or php logs