PHP case sensitivity

I’ve always found the case sensitivity rules in PHP a little mysterious, and also a little hard to find definitive information on. So, for reference sake here they are. Tests based on PHP 5.3 running on Mac OS X:

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.)