PHP Manual . ... A constant is a identifier (name) for a simple value. ... You can define a constant by using the define() -function. ... You can also use the function constant() , to read a constant's value, if you are to obtain the constant's name dynamically. Use get_defined_constants() to get a list of all defined constants. ... If you use an undefined constant, PHP assumes that you mean the name of the constant itself. ... Use the defined() -function if you want to know if a constant is set. ...