When operating in a project context (e.g., having opened a folder rather than a single file), post warnings about PHP items (functions, methods, classes, variables, constants) that are undefined. The purpose is to catch typographical errors in the spelling of names.
While editing, one can usually tell that a function is known because the code is being hinted while one is typing. Similarly, one can tell that a function might be unknown or misspelled if such hinting is not provided. However, it is easy to miss these hints. In a recent example, I typed the function name "styel" instead of "style" and just kept typing without paying attention. The result passed syntax checking. The section of code was not immediately executed and tested so I did not trip an error.
When the file with this error was loaded into JetBrains PhpStorm it was instantly flagged at a warning level (as undefined). When loaded into Rapid PHP, the error was not detected at all.
I realize that this is probably a big feature to implement. My reason for the request is that I want the IDE to help me get my code as clean as possible before it is published. In the case of my "styel" typo, the code was in production with the error. My fault for not testing properly, but something the IDE could have helped prevent.