Drush site-alias via php doesn't return short alias

Question

when I do "drush sa" in terminal I get :

$ drush site-alias --short
@site-drupal
@i3f
@kera
@none

But, when I do it in a php script

passthru("/usr/local/bin/drush sa --short");

I get only

@none @self 

If I omit --short, I get the URIs of the missing aliases !

I just don't get it!

Path: Drush seems to not get access to drushrc aliases...