What Are Constants?
The Unchanging “Variable”
While a “constant” is not a specific data type, it is a type of variable. Specifically, it is a variable that is not changeable. Here are some hard and fast rules about constants:
- Constants are always in UPPER_CASE
- Constants are defined by the programmer
- Constants cannot change within the life of the current script
Some reasons you might want to use constants (but not limited to):
- for unchanging database connections
- for file upload parameters
- for directory pathways
- in general, to assign an unchanging value to a variable to help avoid security holes
Defining a constant
The way we define a constant is with the define() function, and it takes this basic syntax:
define("CONSTANT_NAME", value);
Here’s an example where we define a numeric value to create a maximum upload size for a form:
define("MAX_WIDTH", 1024);
To see it in a browser, you would simply type:
echo MAX_WIDTH;
And it would output to the browser: 1024
Notice how the constant does not have quotes around it in the echo command! If you put quotes around it, it will be read as a string and output: MAX_WIDTH
If we wanted to increase the increment of that constant, we could not do so. If, for instance, you were to type echo MAX_WIDTH++; it would throw an error to the screen because the constant cannot be changed.