Yes, but not in the same straightforward way as with a camera hot shoe. Practical options mentioned are:

• Use an app that syncs the iPhone camera to an external flash, such as iSyncFlash. In this setup, the phone times its exposure to the flash rather than directly controlling the flash in a standard camera/flash way. Reported compatibility may vary, especially on newer phones.
• Use a trigger system such as TriggerTrap Mobile, which can send a sync signal through the phone’s headphone jack when used with compatible adapters/devices.
• If your flash can be set to an optical/wireless slave mode, it may fire when it detects another flash pulse, but this depends on the exact flash version and setup.

So, yes, it can be done in practice, but usually through apps, optical triggering, or external trigger accessories rather than direct native iPhone flash control.