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.
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