The $pattern argument doesn't use /pattern/ delimiters, unlike other regex functions such as preg_match.
<?php
# Works. No slashes around the /pattern/
print_r( mb_split("\s", "hello world") );
Array (
[0] => hello
[1] => world
)
# Doesn't work:
print_r( mb_split("/\s/", "hello world") );
Array (
[0] => hello world
)
?>