selling iPhone; how to erase private data?

I have an old iPhone that i'm going to sell it , in addition, my phone also has some important messages need to deal with , because those are about my bank account, so I tried the factory reset program , and I always thought it was enough to clean them up.
But recently, I read an article on Google.It says that data can't be cleaned up at all after factory reset .
Why ? Is it wrong?

 

I don't know where you got that impression. Doing a factory restore through itunes or erase all content and settings from the iOS menu will clear your phone of everything. Sometimes internal system logs get leftover through option 2, so I always stick to option 1, especially when that pesky "other" folder starts to balloon in size.

 

Are you sure ? but Google has certified this concept, the factory reset is not enough , i have tested it on my iPhone , those deleted data can be retrieved .......

 

If you're that paranoid, then don't sell the device. Smash it with a hammer and recycle the leftovers.

 

Lol......this is not a good idea for a poor man , I hope you can give me other effective advice.

 

Unless you're holding NSA secrets, restore in iTunes and sell it

 

But there is still the possibility of recovery, isn't it ?

 

Not if you're that paranoid, which you clearly seem to be.

 

I think he needs to have it nuked with a hydrogen bomb

 

@Faskerya fill it with junk data to the brim, then factory restore.