While testing MDL-68745 I've seen these params are not working as expected for the English alphabet (this error is unrelated to the patch there because I've been able to reproduce without it).

Steps to reproduce:

1. Create a course and enrol several users.
2. Access to the Participants page.
3. Add "&tifirst=X&tilast=Y" to the end of the current page URL and press enter (replacing X and Y by the first letter of some of the enrolled users in the course).

Expected behaviour:

Only users having X as the first letter in their name and Y as the first letter in their surname are displayed. X and Y letter are marked as selected.

Current behaviour:

All users are displayed (and All are marked as selected).

** The test using Hebrew as site language is worked as expected (following the steps in testin - RTL non-English characters (Hebrew) scenario).