A fork bomb works by creating a large number of processes very quickly in order to saturate the available space in the list of processes kept by the computer’s operating system. If the process table becomes saturated, no new programs may be started until another terminates.

WikiPage : http://en.wikipedia.org/wiki/Fork_bomb

while(1)
pcntl_fork();

Note: Process Control support in PHP is not enabled by default. You have to compile the CGI or CLI version of PHP with –enable-pcntl configuration option when compiling PHP to enable Process Control support.
Currently, this module will not function on non-Unix platforms (Windows).