Q:

How can I specify that my project requres PHP 5.x or HHVM 3.x with Composer?

How can I specify that my project requires PHP 5.x or HHVM 3.x with Composer?
Something as: 
"require":{
  ...
  "php":"~5.3" OR "hhvm":"~3.2" ,
  .....
} 

A:

See here
https://github.com/composer/composer/issues/751
They are working on it!

