capturing echo into a variable

I am calling functions using dynamic function names (something like this)

$unsafeFunctionName = $_POST['function_name'];
$safeFunctionName   = safe($unsafeFunctionName); // custom safe() function

Then I am wanting to wrap some xml around the returned value of the function (something like this):

// xml header etc already created
$result = "<return_value>" . $safeFunctionName() . "</return_value>";

Problem is, sometimes the function returns a value, but sometimes, the function echo's a value. What I want to do is capture that echo into a variable, but, the code I write would need to work either way (meaning, if function returns a value, or echo's a string).

Not quite sure where to start ~ any ideas?

From stackoverflow

• Let me preface this by saying: Be careful with that custom function calling business. I am assuming you know how dangerous this can be which is why you're cleaning it somehow.

  Past that, what you want is known as output buffering:

  function hello() {
      print "Hello World";
  }
  ob_start();
  hello();
  $output = ob_get_clean();
  print "--" . $output . "--";
  

  (I added the dashes to show it's not being printed at first)

  The above will output --Hello World--

  Jon Benedicto : Just a minor point, do you know about the ob_get_clean method?

  Paolo Bergantino : Either would work fine here, no?

  OneNerd : perfect - thanks!

  Jon Benedicto : @Paolo, ob_get_clean just saves an extra line of code.

  Paolo Bergantino : Ah, indeed it does.

• PHP: ob_get_contents

  ob_start(); //Start output buffer
  echo "abc123";
  $output = ob_get_contents(); //Grab output
  ob_end_clean(); //Discard output buffer
  

• In order to use the output value, if present, or the return value if not, you could simply modify your code like this:

  ob_start();
  $return_val = $safeFunctionName();
  $echo_val = ob_get_clean();
  $result = "<return_value>" . (strlen($echo_val) ? $echo_val : $return_val) . "</return_value>";