A constant is a term that identifies a single value. During the script, the value cannot be altered. A valid constant name begins with a letter or underscore (no $ sign before the constant name) and ends with a letter or underscore. Constants, unlike variables, are always global over the whole script.
A constant name is one that begins with a letter or underscore and continues with any number of letters, numerals, or underscores. A constant can never be modified or undefined once it has been defined. You must use the define() function to define a constant, and you just merely mention its name to receive its value.