Constants in PHP

Constants are similar to variables in that they are named memory locations holding certain values; unlike variables, however, once a constant was assigned a value at its declaration, it cannot be changed during script execution.

Constants are used to store some frequently used numbers or strings. For example, your site may be programmed for 640-pixel wide pages and you want to switch to using 800-pixel wide pages. If you used number 640 explicitly in your code, you will have to find all instances it was used in the code and change it. Even though this task can be automated, there is no guarantee that you will find all the numbers that need to be changed or not change a number 640 referring to something other than the page width. Instead of using the number 640 explicitly, you can declare a constant, for example $PgWdth, at the beginning of the file, set it to 640, and then use the constant throughout the file wherever you need to use number 640. Then, if you need to change 640 to 800, all you need to do is to reassign the value of the constant $PgWdth to 800 once at the beginning of the file.

I recommend always using constants or at least variables if a number or a string is used more than once in the code. These constants and variables can be stored in a separate file, which can then be included into the PHP files using these constants or variables. Based on my personal experience, I can tell that using constants can make software maintenance and modification significantly easier.