According to W3 Schools: "A constant is an identifier (name) for a simple value. The value cannot be changed during the script. A valid constant name starts with a letter or an underscore (no $ sign before the constant name)." in my own words: A constant in PHP is like a box where you store a value that never changes while the script is running. Unlike variables (which use a $ sign and can change), constants stay the same once you set them.