Timoh has posted a look at random number generation to his site, focusing on one of the many methods to produce truly random number - using /dev/(u)random (available on Unix-based filesystems).
How would you gather cryptographically secure random bytes in your PHP application? This is actually quite a good question. It used to be, and seems, it still is not that uncommon to just simply call mt_rand() function to get the job done creating user's "initial password", for example. A bit more experienced reader will notice there is a security bug. [...] But actually only a few [functions to get random values] can be recommended for security sensitive purposes. And now I'm not talking about openssl_random_pseudo_bytes().
He starts with a look at openssl_random_pseudo_bytes and why there might be something wrong with its use - mainly that OpenSSL has had its own share of security issues in the past. Of the two random resources he recommends /dev/urandom as it's less blocking and more useful for web applications. He recommends the RandomCompat library if you need to take this random data and transform it into integers (with one caveat).