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Just follow this quick guide to erase iPhone personal data:
Settings > General > Reset > Erase all content and settings. Do that about 3 times. Then put it into DFU mode and restore if you're still worried. It may be recoverable by some iPhone data recovery software like Tenorshare iPhone Data Recovery
There is no need to waste your money on third party software, just go to SETTINGS>GENERAL>RESET and then select Erase All Content and Settings. Here are Apple’s instructions on “What to do before selling or giving away your iPhone, iPad, or iPod touch.”