FYK (updated)

Case sensitive (both user-defined and PHP defined)

• variables
• constants ->>check Amendment 1
• 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.)

In php.net

Basics

Variables in PHP are represented by a dollar sign followed by the variable’s name. The variable name is case-sensitive.

Variable names follow the same rules as other labels in PHP. A valid variable name starts with a letter or underscores, followed by any number of letters, numbers, or underscores. As a regular expression, it would be expressed thus: '[a-zA-Z_\x7f-\xff][a-zA-Z0-9_\x7f-\xff]*'

Amendments

1. Class constants are always case-sensitive. Global constants declared with const are always case-sensitive. It should be noted that this applies only to the shortname of the constant, while namespaces in PHP are always case-insensitive. Constants declared with define() are case-sensitive by default

Some useful Links

1. Userland Naming Guide
2. Why are functions and methods in PHP case-insensitive?
3. Are PHP functions case sensitive?
4. Are PHP keywords case-sensitive?
5. Are PHP function names case-sensitive or not?
6. Source of PHP Case Sensitive