Question

In other words, can fn() know that it is being used as $var = fn(); rather than as fn();?

A use case would be to echo the return value in the latter case but to return it in the former.

Can this be done without passing a parameter to the function to declare which way it is being used?

Answer 1

Many PHP functions do this by passing a boolean value called $return which returns the value if $return is true, or prints the value if $return is false. A couple of examples are print_r() and highlight_file().

So your function would look like this:

function fn($return=false) {     if ($return) {         return 'blah';     } else {         echo 'blah';     } } 

Any other way will not be good programming style.