Are PHP keywords case-sensitive?

Question

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

Answer 1

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.)