Identifiers are names made up by the programmer to refer to variables, functions, or types. Identifiers should suggest their use.

PHP is usually case sensitive - that is, upper case letters are considered to be different from the corresponding lower case letters in identifiers and keywords.

Identifiers are used to refer to:

• variables (when prefixed with $)
• classes
• constants
• functions
• formal parameters

The limitations are:

• Reserved words cannot be used as identifiers.
• In php, syntactic keywords are all reserved words.
• The same identifier can be used in different scopes to refer to different elements.
• Identifiers can be any length.
• Although identifiers can begin with an underscore (_), this should usually be avoided: compilers may generate identifiers beginning with an underscore for their own purposes, and user defined variables beginning with an underscore could conflict.