1.函数名称不区分大小写
Function names are case-insensitive, though it is usually good form to call functions as they appear in their declaration.
http://www.php.net/manual/en/functions.user-defined.php
2.变量名，常量名区分大小写
Variables in PHP are represented by a dollar sign followed by the name of the variable. The variable name is case-sensitive.
http://www.php.net/manual/en/language.variables.basics.php
3.类名称，方法名不区分大小写