It will charge. It's not dangerous. That power adapter is designed to supply UP TO 87W. If you plug a 15" tbMBP into it, it will draw all 87W provided. If you plug a 13" tbMBP into it, the MBP will only draw 61W, even though the adapter can provide 87W. Same for you iPhone. It will draw the amount of power it needs (~10W, IIRC), and the adapter has no problem providing only that amount of power.
However, it is interesting to note that the USB plug adapters that come with iPhones provide only 5W, iPhones are capable of pulling more power, and will charge faster when plugged into an iPad (10W) or tbMBP USB plug adapter.