I suppose forking PHP is as realistic as forking Perl :)
Also i can imagine two major reasons PHP is not forked:
1) You have a small committed developerbase which is too much invested in the ecosystem (Zend and others).
2) Every other user (aka webdeveloper and PHP developers that want a major change) in the ecosystem may rather move on to another language then try to fork it. In the end: What what you do with a PHP fork that doesn't move in the direction of Ruby, Python and Co.?
Also i can imagine two major reasons PHP is not forked:
1) You have a small committed developerbase which is too much invested in the ecosystem (Zend and others).
2) Every other user (aka webdeveloper and PHP developers that want a major change) in the ecosystem may rather move on to another language then try to fork it. In the end: What what you do with a PHP fork that doesn't move in the direction of Ruby, Python and Co.?