Echoing session variables in php

I know that in php I can put a variable name inside a quoted string when I use echo, but I apparently can't do this with a session variable. Can anyone explain why?

Here is the code, with the "offending" php commented out:

session_start();
$test = 100;
$_SESSION['test'] = 200;
?>

  
    
  
  
  
");?>
  
");?>
  
");?>
  
");?>
  

And the output looks like this:

100

100

200

But if I uncomment the offending code line:

  
");?>

I get no output and the following error:

Parse error: syntax error, unexpected T_ENCAPSED_AND_WHITESPACE, expecting T_STRING or T_VARIABLE or T_NUM_STRING in - on line 14

So I can go on my merry way knowing how to do it correctly (just keep the session variable outside of the double quotes), but I would really like to understand why this doesn't work for session variables.

Thanks!

Answer

Inside a double-quoted string you must enclose a complex variable (array or object property) in {}:

");?>

This isn't an issue with $_SESSION specifically, but any array accessed by quoted keys. Note, that you can include a numerically indexed array value with wrapping in {}, as in "echo $array[2] is two";