A PHP Facebook website and FB Messenger app alike - give or take.

Hi there, I need a website in PHP language very similar to Facebook and an FB alike messenger app compatible with iOS/Android/Windows platforms. The site and messenger must be fully synchronised. There are hundreds of FB similar scripts on the internet some of them even free, but I would like some modifications to be done. I don't need most of the gadgets that FB is having on their site, but I need the main features such as the chat system with all its functions, posting, sharing, the search functionalities, notifications, instant search suggestions, settings, voice and video chat, posting videos directly from the device...etc The messenger app just like the FB messenger with all its functions, notifications, settings...etc.. The site could have same layout as FB or slightly different. So all in all a site that is very similar to FB +/- few modifications. I can also buy a ready-made script which is very similar to FB but will need to be modified. I presume is best and quicker if I give you the whole script and you do the modifications to suit my needs? Please let me know which is the better solution. I haven't got everything ready yet as to how exactly the site will function, but I am almost there. I just need your estimated quote based on the above info so that I can compare to others. Many thanks.