An expression is code that is evaluated to produce a value. An expression is made by combining a data value with an operator. Both Javascript and PHP support operators for combining numbers to form arithmetic operations (+,-,*,/).
If the result of an expression is not stored in a variable, the value is lost:
0 + 0;
Once a value is stored in a variable, it can be used more than once in a program.
var number_zero = 0 + 0;
number_zero;
number_zero;