Perfect cut and concatenate video script using ffmpeg

We need a script to cut videos (mp4) and concatenate them with ffmpeg. So the requirement is to be able to cut video parts with a start time and duration. But be carefull, start times are not on keyframes so this means you'll need to do some manipulations with video in order to be able to cut on the exact frame needed. Please explain in your answer that you fully understood this issue. Example: [login to view URL] -> 00:00:08.234 with 00:00:02.435 duration File2.mp4-> 00:00:18.423 with 00:00:04.121 duration File3.mp4-> 00:00:06.420 with 00:00:03.210 duration Then concatenate those three files files in order to have only one video. The script should be delivered in PHP using ffmpeg.