PHP - Manual: IntlChar::foldCase (官方文档)
IntlChar::foldCase — Perform case folding on a code point
IntlChar::FOLD_CASE_DEFAULT] ) : mixed
The given character is mapped to its case folding equivalent; if the character has no case folding equivalent, the character itself is returned.
Returns the Simple_Case_Folding of the code point, if any; otherwise the code point itself.