The issue is the Tristar chip (controls charging and USB circuit) needs to be replaced via micro-soldering.
A classic symptom is the battery going flat then the phone does not charge flat batteries.
Another dead giveaway is not seeing an apple logo when plugging in the charger with the battery unplugged.
Like others said make sure MFi certified cables are being used because they have surge protection built in them via the small chip in these cables.
Unfortunately iPhones don't have surge protection so the Tristar chip basically acts like a fuse that gets damaged.