Syntax

You can define a constant by using the define()-function. Once a constant is defined, it can never be changed or undefined.

Only scalar data (boolean, integer, float and string) can be contained in constants.

You can get the value of a constant by simply specifying its name. Unlike with variables, you should not prepend a constant with a $. You can also use the function constant() to read a constant's value if you wish 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, just as if you called it as a string (CONSTANT vs "CONSTANT"). An error of level E_NOTICE will be issued when this happens. See also the manual entry on why $foo[bar] is wrong (unless you first define() bar as a constant). If you simply want to check if a constant is set, use the defined() function.

These are the differences between constants and variables:

• Constants do not have a dollar sign ($) before them;

• Constants may only be defined using the define() function, not by simple assignment;

• Constants may be defined and accessed anywhere without regard to variable scoping rules;

• Constants may not be redefined or undefined once they have been set; and

• Constants may only evaluate to scalar values.

Example 8-1. Defining Constants <?php define("CONSTANT", "Hello world."); echo CONSTANT; // outputs "Hello world." echo Constant; // outputs "Constant" and issues a notice. ?>