The names of user-defined classes and functions, as well as built-in constructs and keywords such as echo, while, class, etc., are case-insensitive. Thus, these three lines are equivalent:
echo(“hello, world”);
ECHO(“hello, world”);
EcHo(“hello, world”);
Variables, on the other hand, are case-sensitive. That is, $name, $NAME, and $NaME are three different variables