Returns string with all alphabetic characters converted to lowercase.
Note that 'alphabetic' is determined by the current locale. This means that in i.e. the default "C" locale, characters such as umlaut-A (Ä) will not be converted.
注: 這個函數可安全的使用於二進位。
See also strtoupper(), ucfirst(), ucwords() and mb_strtolower().