PHP_Fork class. Wrapper around the pcntl_fork() stuff with a API set
like Java language. Practical usage is done by extending this class,
and re-defining the run() method.

This way PHP developers can enclose logic into a class that extends
PHP_Fork, then execute the start() method that forks a child process.
Communications with the forked process is ensured by using a Shared
Memory Segment; by using a user-defined signal and this shared memory
developers can access to child process methods that returns a
serializable variable.

The shared variable space can be accessed with the two methods:
- void setVariable($name, $value)
- mixed getVariable($name)

$name must be a valid PHP variable name;
$value must be a variable or a serializable object.

Resources (db connections, streams, etc.) cannot be serialized and so
they're not correctly handled.

In PEAR status of this package is: beta.