An identifier is simply a name. In PHP, identifiers are used to name variables, functions, constants, and classes. The first character of an identifier must be an ASCII letter (up-percase or lowercase), the underscore character (_), or any of the characters between ASCII 0x7F and ASCII 0xFF. After the initial character, these characters and the digits 0–9 are valid
Variable names
Variable names always begin with a dollar sign ($) and are case-sensitive. Here are some valid variable names
Here are some illegal variable names:
$not valid
$|
$3wa
These variables are all different due to case sensitivity:
$hot_stuff $Hot_stuff $hot_Stuff $HOT_STUFF
Function names
Function names are not case-sensitive.Here are some valid function names:
tally
list_all_users
deleteTclFiles
LOWERCASE_IS_FOR_WIMPS
_hide
These function names refer to the same function:
howdy HoWdY HOWDY HOWdy howdy
Class names
Class names follow the standard rules for PHP identifiers and are also not case-sensitive. Here are some valid class names:
Person
account
The class name stdClass is reserved
Constants
A constant is an identifier for a simple value; only scalar values—Boolean, integer, double, and string—can be constants. Once set, the value of a constant cannot change. Constants are referred to by their identifiers and are set using the define() function:
define(‘PUBLISHER’, “O’Reilly & Associates”);
echo PUBLISHER;