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: 本函数可安全用于二进制对象。
See also strtoupper(), ucfirst(), ucwords() and mb_strtolower().