PHP Pocket Reference, 2nd Edition (2009)
An expression is the basic building block of the language. Anything with a value can be thought of as an expression. Examples include:
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 printare identical: they output data.