As I understand, it was a conscious decision not to add this function when str_contains was created. The reason is that case sensitivity is locale-dependent, and for such use cases, mbstring extension is better [1] & [2]. Do you think that locale is a concern here, and if not, why? Would it be a good idea to add mb_str_icontains instead?
If you're going to propose an RFC for this, it would be a good idea to explain what the real life use case for it is. While str_contains is very useful for checking the existence of a byte-string within another byte-string, a case-sensitive check doesn't seem to have much use. [1]: https://stackoverflow.com/a/63121809/1839439 [2]: https://wiki.php.net/rfc/str_contains#case-insensitivity_and_multibyte_strings
