Basically names of user-defined functions, classes, built-in constructs and keywords such as echo, while, class, etc., are case-insensitive. However; variables on the other hand, are case-sensitive.
In the example below, all three echo statements below are legal (and equal):
However; variables on the other hand, are case-sensitive. That is, $variable, $VARIABLE, $VariaBle and $VAriable are four different variables.
This will produce following result