@Ann, I would have associated this rule to the "Reliability" SQALE characteristic. I'm using the "Changeability - Data related changeability" mainly when a design flaw may prevent the developer from changing a structure of data.
"_{color:grey}Global variables are a useful construct, but they should not be abused{color}_.
A function can access the global scope through the global keyword.This practice considerably reduces the readability of a function. The global variable should be passed as a parameter of the function."
. About the tag I agree, convention suits well for me.
. Also, maybe we could the add in this rule the use of $GLOBALS array, see: \http://jira.codehaus.org/browse/SONARPHP-131, sorry for realising this only now.