PHP vs. Other Languages in Hadoop/MapReduce implementations, and in the Cloud generally
The reason is PHP lack of support for multi-threading and process communication.
I'm beginning to learn some Hadoop/MapReduce, coming mostly from a PHP background, with a little bit of Java and Python. 
But, it seems like most implementations of MapReduce out there are in Java, Ruby, C++ or Python. 
I've looked, and it looks like there are some Hadoop/MapReduce in PHP, but the overwhelming body of the literature seems to be dedicated to those 4 languages.
Is there a good reason why PHP is a 2nd class language in cloud computing projects like those that involve Hadoop/MapReduce? This is particularly surprising, considering that, outside of cloud computing world, PHP seems like its the most commonly supported language, to the detriment of the 3 above (sans C++) languages. 
If this is arbitrary--if PHP is just as good at handling these operations as, say, Python, what libraries/projects should I look into?