PHP - detect if a function is used to assign a value to a variable

so I have 2 functions like this:

function boo(){
  return "boo";
}

and

function foo(){
  echo "foo";
}

the fist one will return a value, and the 2nd one will output something to the screen directly.

$var = boo();
foo();

How can I merge these 2 functions into one, and somehow detect if it's being called to output the result 开发者_开发知识库to the screen, or if it's called for getting the return value? Then choose to use return or echo...

---

function boo_or_foo ($output = false) {
    if ($output) {
        echo "fbo";
    } else {
        return "foo";
    }
}

But whats the benefit against just using one function (boo()) and echo it yourself?

echo $boo();

---

Well, a function should only do one thing, so typically you would have two functions. But, if you would like to combine them you can just check if is set:

function boo($var=null){
  if(isset($var)) echo $var
  else return "boo";
}

---

well return true in the function that prints then yo just do

function foo(){
  echo "foo";
  return true;
}

    if(foo()){
    echo "foo did print something";
    }else{
    echo "nope foo is broken";
    }

---

I wanted to achieve the same effect. In my case I have functions that produce HTML which I want echoed directly sometimes (when an Ajax call is being made), or returned (when a call is made from another script).

For example, a function that creates a list of HTML <option> elements - listOfOption($filter). When one of my pages is first created, the function is called and the result is echoed in place:

<?= listOfOption($var) ?>

But sometimes the same data needs to be retrieved in an Ajax call:

http://site.com/listOfOption.php?parameter=2

Instead of writing two different scripts or specifying the behaviour in a parameter, I keep listOfOption($filter) in its own file like this:

if (__FILE__ == $_SERVER['SCRIPT_FILENAME'])
{
    echo listOfOption($_REQUEST['parameter']);
}

function listOfOption($filter)
{
    return '<option value="1">Foo</option>';
}

This way if the call is from another script, it returns the data; otherwise it prints the data.

Note that if a parameter isn't passed to the function I wouldn't have to do this, I could live with echoing the data always and replacing the <?= listOfOption() ?> invocation with <? listOfOption() ?> to keep things clear.