Are PHP keywords case-sensitive?

Question

For example, is the following program meaningful, and if so what should it print?

Answer 1

No. Keywords are case-insensitive. Lerdorf et al., Programming PHP, page 17:

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");

Answer 2

1. No, Keywords are not case-sensitive.
2. Class name & function name also not case-sensitive.

Answer 3

Case sensitive (both user defined and PHP defined)

• variables
• constants
• array keys
• class properties
• class constants

Case insensitive (both user defined and PHP defined)

• functions
• class constructors
• class methods
• keywords and constructs (if, else, null, foreach, echo etc.)