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I'm getting my daughter an emergency alert bracelet because we're traveling across the US soon. On the bracelet I'm getting my website address for the person who finds/kidnaps my kid.

When every ANYONE visits the unique URL, I want it to register their IP, and as much other personally identifying data as possible, then send that to me as an email.

This should tell me where they are located at a bare minimum, but in the even that they are criminals, I'd like to have enough information to help the police track them down faster.

I was thinking a $_SERVER dump via PHP, and possibly some tricky JavaScript methods.

What specifically should I implement using PHP and Javascript to track down the visitor?

user3657553
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    You could try to get the (hopefully legit) [user IP](http://stackoverflow.com/questions/15699101/get-the-client-ip-address-using-php), but that's the only "useful" thing you can give to the police. Getting the (hopefully legit) [User Agent](http://stackoverflow.com/questions/10243841/how-to-get-user-agent-in-php), you can determine if they're using a mobile or a PC, if that can help. – blex Jul 16 '15 at 19:12
  • fingerprinting != gathering personal info. that said, you can get a lot of stats about the machine using http://danml.com/js/blackbox.js that aside, i can't imagine a kidnapper or thief going to your website, but that's me... – dandavis Jul 16 '15 at 19:13
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    Short of hacking the browser, there's not much _useful_ information you can get from a browser. You can get the browser and operating system manufacturer, what plugins and fonts are loaded, and what ISP they are using. That doesn't tell you much about the operator's identity, and if someone is into serious crimes like kidnapping, they surely will not use their own computer. Do you have a particular reason to worry that your child is at risk of kidnapping? I wonder if you just need some relaxation exercises. – halfer Jul 16 '15 at 19:15
  • Thanks Halfer and others. I will look at the blackbox JS. We are just traveling for about 2 months in an RV, stopping at many parks, malls, etc, and wanted to be extra safe. We will probably use the website page for years to come for all of our kids. – user3657553 Jul 16 '15 at 19:17
  • @dandavis How would you try to get your ransom, then? – blex Jul 16 '15 at 19:18
  • i think the IP is the main thing of immediate value, but a system fingerprint could help if (heaven forbid) there is an incident and (heaven forbid) it goes bad, the more info you have that could tie a machine to the crime, the more convincing the evidence will be to a jury. in that line of thought, font fingerprinting is really good; it can uniquely ID about 90% of machines. – dandavis Jul 16 '15 at 19:28

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The reason why web applications are blowing up is because how secure they are. Browsers unfortunately will not give you access to much information, however you can access their IP address and Geolocation.

I think the most data you will be able to get is their Long/Lat and IP address, however they would need to allow the site to grab their Geolocation.

http://www.w3schools.com/html/html5_geolocation.asp

Eric E
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  • _"Hi, dear kidnapper. Would you like to share your current location with us?"_ – blex Jul 16 '15 at 19:21
  • *"Hello, Yes let me click on Allow. These geolocations estimates are not accurate anyways."* – Eric E Jul 16 '15 at 19:23
  • _"Thank you, have a wonderful day."_ Jokes aside, it's pretty darn accurate when I use it, whether it's on phone or desktop. – blex Jul 16 '15 at 19:24
  • Is it really? I used it for an application a while ago, and would always get a rough estimate when using a Desktop computer, I was only able to get accurate reading when on a mobile phone. – Eric E Jul 16 '15 at 19:26
  • I just tried it [here](http://www.w3schools.com/html/html5_geolocation.asp), and it located me 15 feet from where I am, but my Internet provider has a small building across my street, maybe that's why. – blex Jul 16 '15 at 19:28
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    Might be just luck :S I tried and it located me about 3 miles from where I am... I am using a laptop. – Eric E Jul 16 '15 at 19:30