Webmaster in a Nutshell, Third Edition

Expressions

An expression is the basic building block of the language. Anything with a value can be thought of as an expression. Examples include:

5
5+5
$a
$a==5
sqrt(9)

By combining many of these basic expressions, you can build larger, more complex expressions.

Note that the echo statement we’ve used in numerous examples cannot be part of a complex expression because it does not have a return value. The print statement, on the other hand, can be used as part of complex expression—it does have a return value. In all other respects, echo and print are identical: they output data.