Setting a Variable in PHP for use in rules
The Rules module doesn't seem to provide this feature. 

I also tried using a rule set with a variable, but the variable was reset between each action. I tried adding return $minimum_reached at the end of each rule executing PHP code, but it didn't help.

The only way seems to use drupal_static(), if you are using Drupal 7 (e.g. with drupal_static('mimimum_reached', 'default value used when the variable is not set')), or a global PHP variable, if you are using Drupal 6.