T_ENCAPSED_AND_WHITESPACED is returned when parsing strings with evaluated content, like "some $value" or this example from the Strings reference page:
<?php
echo <<<EOT
My name is "$name". I am printing some $foo->foo.
Now, I am printing some {$foo->bar[1]}.
This should print a capital 'A': \x41
EOT;
?>
This last example is tokenized as:
T_ECHO 
  echo
T_WHITESPACE 
  %20 (a space character)
T_START_HEREDOC 
  << 
T_ENCAPSED_AND_WHITESPACE 
  My name is "
T_VARIABLE
  $name
T_ENCAPSED_AND_WHITESPACE    
  ". I am printing some 
T_VARIABLE    
  $foo
T_OBJECT_OPERATOR    
  ->
T_STRING    
  foo
T_ENCAPSED_AND_WHITESPACE    
  . Now, I am printing some 
T_CURLY_OPEN    
  {
T_VARIABLE    
  $foo
T_OBJECT_OPERATOR    
  ->
T_STRING    
  bar
(terminal)
  [
T_LNUMBER    
  1
(terminal)
  ]
(terminal)
  }
T_ENCAPSED_AND_WHITESPACE    
  . This should print a capital 'A': \x41
T_END_HEREDOC
  EOT
(terminal)
  ;