New Relic picks up all errors PHP is configured to throw. So if you don’t want New Relic to pick up your E_WARNING errors you need to tell PHP not to log E_WARNING errors.
This would be achieved quite simply by modifying your php.ini file
In PHP code terms this would be
error_reporting(E_ALL & ~E_WARNING);
In your php.ini you would add
error_reporting = E_ALL & ~E_WARNING
This tilda tells PHP not to log an error of that type. So the above statement is log ALL errors except Warnings. This means you can define all levels you don’t want to log and PHP will not log them and New Relic will therefore not see them to report them.
Just for extra information, Even if PHP is reporting them, New Relic will never show in its dashboards error levels E_NOTICE or E_USER_NOTICE.
I hope this helps you resolve your issue.