The define() function is used to define constants. Once a constant is defined, it can not be changed or undefined.
An example of defining constants:
<?php define("CONSTANT", "Hello!"); echo CONSTANT; ?>
Constant names and any other PHP tags follow the same naming convention. Valid constant names start with a letter or an underscore followed by any letter, number, or underscore.
Constants are case-sensitive by default, and conventionally constant identifiers are always uppercase and can not be changed during script execution.
Define the difference between a constant and a defined variable:
- Constant without a dollar sign ($)
- Constants can only be defined with define() function, but not through the assignment statement
- Constants can be defined and accessed anywhere without regard to the rules of the variable scope
- Constants can not be redefined or undefined once they are defined
- The value of a constant can only be scalar
PHP built-in a large number of predefined constants, the specific can refer to the PHP manual.