PHP variables are used to store data values that can be used and manipulated throughout a script. They are declared using the dollar sign ($) followed by the variable name. PHP variables are dynamic, which means they can hold different data types, such as strings, integers, floats, and booleans, based on the value assigned to them at runtime. PHP constants, on the other hand, are values that cannot be changed once they have been defined. They are typically used to store values that do not need to be modified throughout the script’s execution, such as database connection details or configuration settings. Constants are defined using the define() function, which takes two arguments: the name of the constant and its value. Once defined, constants can be used throughout the script by simply referring to their name.