The IDE is using PHP language level 5.5 and I don't want it to recognize mssql library because I'm on window environment.
mssql library functions will cause errors on php 5.5 application and these errors can't be detected by IDE
Can we remove mssql library recognition?
You CANNOT tell IDE to "forget" any specific class ... unless you alter PhpStorm's installation and physically remove stub file for it (which you will have to do for every new version).
You could re-declare the class/functions yourself instead and add @deprecated
tag to PHPDoc comments for them -- this will tell IDE to mark them as deprecated and warn you (if corresponding inspection is turned ON -- Settings | Inspections | PHP | General | Deprecated
)