是否有必要将值放入变量？

Question

I have been asked about this simple question by a student and I was greatly troubled because I cannot provide a sufficient answer to this. What I understood is that, a variable is just a container that holds a value. Aside from that, I cannot provide a logical explanation to them.

example:

I want to check if an age is minor or not. By using a variable, I will make it this way:

<?php 
$age = 30;
if( $age > 18)
   echo 'You are no longer a minor';
?>

But, without using a variable, I can make it this way:

<?php  
if( 30 > 18)
   echo 'You are no longer a minor';
?>

If the two scripts produce the same output, then why should we put it in a variable, for in fact, we can lessen the code of we'll not use a variable. And also, Is it necessary to put a value into a variable?

Does it affect the memory of a computer?

Answer 1

The answer here is that age in any valid usage will come from some form of input (user input, database select, webservice call etc) which would be stored in a variable in order to proceed with the rest of the logic. Your example here tries to falsify the input by setting the variable to a predefined value, which is all cool and dandy for examples, but in normal situations you would read that value from somewhere, hence the variable