Grav admin panel - errors about "mysqli already loaded"

Every time I access the admin panel (and only the admin panel -- no issues on my actual site), the following things happen. Any insight is appreciated...

1) I get a red error box saying "Fetch failed: Crikey! There was an error...", with more text below than the box can fit. Site inspection reveals the following:

admin.min.js?0495866071:4   Crikey! There was an error...
[some CSS]
Whoops \ Exception \ ErrorException (E_CORE_WARNING) Module 'mysqli' already loaded
Whoops\Exception\ErrorException thrown with message "Module 'mysqli' already loaded"
Stacktrace:
#0 Whoops\Exception\ErrorException in Unknown:0
COPY
Stack frames (1):
0
Whoops\Exception\ErrorException
Unknown0
Unknown
[lots of JS and other stuff]

and

admin.min.js?0495866071:1   POST http://[site]/admin 500 (Internal Server Error)

2) I get a log entry saying:
---[2016-09-27 10:22:15] grav.CRITICAL: Module 'mysqli' already loaded - Trace: #0 [site]/vendor/filp/whoops/src/Whoops/Run.php(363): Whoops\Run->handleError(32, 'Module 'mysqli'...', 'Unknown', 0) #1 [internal function]: Whoops\Run->handl eShutdown() #2 {main} [] []

That's a problem with your PHP setup, and would give an error with any PHP script. Grav doesn't even use mysqli it's just reporting a problem with the PHP configuration.