How to make TLS connection from PHP in web server, and safely

Question

Suppose I have some PHP code running inside a web server, for example, running a simple CakePHP app. From this app, I want to occasionally make a TLS connection out to some server to exchange some data. How is this typically done? (I have little experience with PHP.)

What PHP plug-ins or libraries or whatever, are recommended to accomplish TLS connections? Where are some good places to start looking? (My initial Google searches give me a huge ratio of noise to signal.)

What are the security issues involved with having the X509 private key sitting on the web server? To make a TLS connection out, I'm going to need both the X509 certificate and the corresponding private key, as perhaps PEM files, or DER files, or in a Java keystore, or whatever. Is that private key going to be sitting somewhere vulnerable under the web root? How is this typically handled? What are the security issues, specifically, of securing the private key that is to be used to make a TLS connection out? What are the best practices?

Edit: I forgot to mention that the server that the PHP app connects to requires a client certificate to be presented. The client does need the private key to connect. Also, the server I am connecting to is not an HTTP server -- I just need to read/write on a plain socket connection to the server (over the SSL/TLS connection).

Answer 1

TLS is the proper name, however most people still call it SSL. In PHP you can make this connection using CURL.

With a TLS/SSL client you only need the public key to verify a remote host. This public key is just that public, it doesn't matter if it gets leaked to an attacker. On the server side Apache has access both the public and private key. These keys are protected using normal file access privileges. Under *nix systems these keys are commonly stored somewhere in /etc/, owned by the Apache process and chmod 400 is best.

The easiest authentication method for clients would be a simple username/password. You can use SSL to authenticate both the client and server. This would require you to store the private key somewhere where your PHP app has access, ideally outside of the webroot with a chmod 400, but you could easily rename it to .php or put it in a folder with a .htaccess that contains deny from all. On the server side you can verify the clients certificate using these environmental variables.

If you just want a TLS connection and not an HTTPs. Then you can use stream_context_set_option by setting the Context Options:

<?php 
$context = stream_context_create(); 
$result = stream_context_set_option($context, 'ssl', 'local_cert', '/path/to/keys.pem'); 
// This line is needed if your cert has a passphrase
$result = stream_context_set_option($context, 'ssl', 'passphrase', 'pass_to_access_keys'); 

$socket = stream_socket_client('tls://'.$host.':443', $errno, $errstr, 30, STREAM_CLIENT_CONNECT, $context); 
?>