PHP case-sensitivity

• 变量名区分大小写

• 常量名默认区分大小写

• 函数名、方法名、类名不区分大小写

  BUT！https://bugs.php.net/bug.php?id=60833 parent,self,static
  “parent” and “self”, “static” LSB tokens behave inconsistent when it comes to case-sensitivity. Class names in PHP are case-insensitive, but these three keywords aren’t always, as the parser sees them as raw T_STRINGs.

• 魔术变量不区分大小写,但默认大写

• null、true、false不区分大小写