House of Technology

Lexical Structure:
The lexical structure of a programming language is the set of basic rules that governs how you write programs in that language. It is the lowest-level syntax of the lan- guage and specifies such things as what variable names look like, what characters are used for comments, and how program statements are separated from each other.

Case Sensitivity:
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.