PHP case sensitivity issue

1. Case sensitive
1. Variable names are case sensitive
All variables are case sensitive, including ordinary variables and $_GET,$_POST,$_REQUEST,$_COOKIE,$_SESSION,$GLOBALS,$_SERVER,$_FILES, $_ENV, etc.;

2. Magic constants are not case-sensitive, uppercase is recommended
including: __LINE__, __FILE__, __DIR__, __FUNCTION__, __CLASS__, __METHOD__, __NAMESPACE__.