How to completely reset an iPhone?

[Zahid TOG]

How to completely reset an iPhone?

[STEVEN123456]

Connect your device to your computer using the cable that came with it.
Select your iPhone, iPad, or iPod when it appears in iTunes.
In the Summary panel, click Restore.
Click Restore again to confirm that you want to restore your device to factory settings and delete all data and content.

[Shellyb]

From Apple:  https://support.apple.com/en-us/HT201351

Follow these steps:

    If you paired an Apple Watch with your iPhone, unpair your Apple Watch.
    Back up your iOS device.
    Tap Settings > iCloud. Scroll down and tap Sign Out. In iOS 7 or earlier, tap Delete Account.
    Tap Sign Out again, then tap Delete from My iPhone and enter your password.
    Go back to Settings and tap General > Reset > Erase All Content and Settings. If you turned on Find My iPhone, you might need to enter your Apple ID and password.
    If asked for your device passcode or Restrictions passcode, enter it. Then tap Erase [device].
    Contact your carrier for help transferring service to a new owner. If you aren't using a SIM card with your device, you can contact them to get help transferring service to the new owner.

[Geek-9pm]

Then you are asking for a 'forensic' erase o the iPhone.
Are you trying to hide from the FBI?
Here is why I made the above remark. Whenever somebody comes to this forum ans asks about doing a full wipe out of data on a computer, phone or other data-storage device, we become suspicious ans to why the average person would ever want to do such thing.
To fully wipe a device to forensic clean does not make it work any better. You old data is not going to pop up and get into anything. You new girlfriend will not get the love letters you sent to your ex.   

[Allan]

Then you are asking for a 'forensic' erase o the iPhone.
Are you trying to hide from the FBI?
Here is why I made the above remark. Whenever somebody comes to this forum ans asks about doing a full wipe out of data on a computer, phone or other data-storage device, we become suspicious ans to why the average person would ever want to do such thing.
To fully wipe a device to forensic clean does not make it work any better. You old data is not going to pop up and get into anything. You new girlfriend will not get the love letters you sent to your ex.   

WE don't become suspicious, you do. It is none of our business why someone wants to reset a phone and your comments are uncalled for. If you don't want to provide assistance, don't post anything. Stop being the Internet police.

[Geek-9pm]

Allan, I got the point.   
Back to topic. Here is a method that might work, but is not perfect.
He could erase as much data as he can and then put new bogus data on the iPhone. The bogus stuff with overwrite some of the same memory area of the data marked for deletion.. Any  OS will over write data marked for deletion as the storage device becomes near full. After that, one could erase the bogus data. Any later attempt to recover data using covert methods will have a lot of bogus data and maybe some of the original data.

That is the best answer I can think of. Otherwise just destroy  the phone

EDIT: Here is a link to a site that claims to have an way to do it.
https://support.apple.com/en-us/HT201351
I have not done . The site claims to be part of Apple support.