The PHP core lacks any cryptographically secure pseudorandom number generators.
Therefore, applications that need crypto level randomness should rely either on
randomness provided through some operating system interface or through some
PHP functions available in some extensions such as the openssl and mcrypt 
extensions.

However, these extensions may not be available in all PHP installations and
furthermore, older PHP versions had buggy implementations of these functions
which sometimes caused them to be painfully slow. Thus, even when using these
functions one should to be aware of the PHP version and OS used.

Finally, in installations that lack openssl or mcrypt extensions and no 
randomness can be obtained from the operating system, we need a way to
extract sufficient randomness for cryptographic purposes. For this purpose
we exploit the fact that due to schedule and other non deterministic events
the time that a CPU intensive operation will take will vary from one execution
to another. Therefore counting the time such events will take contains some
bits of entropy (although we need to be extremely cautious on how many bits 
we assume to extract from each execution).

secure_random_bytes function implements a secure randomness generator using 
the ideas described above. In summary, it first attempt to use one of the PHP 
functions from the openssl and mcrypt extensions if the PHP version used does
not contain a bug in the implementation of these functions. Otherwise, it 
attempts to gather entropy either from /dev/urandom or by measuring the time 
needed to complete the calculation of a certain number of sha1 hashes. In 
order to keep the running time low while keeping our security level, we define
a constant time that we want our operation to take on average. This time 
should not be too large in order to keep the running time low, nor too low
in order to have a sufficient time variation between different executions. 

We tested this function in Windows XP/2003, Sun Solaris and Debian/Ubuntu Linux,
where it provided as many bits of entropy as requested by the user while keeping
the running time relatively low. 

This software was written based on research funded by the ERC project CODAMODA
and is released under the New BSD License.