<?php
$sv = array("small", "larger", "a big string", "it is a string thing");
print_r($sv);
?>
Array
(
  [0] => small
  [1] => larger
  [2] => a big string
  [3] => it is a string thing
)

<?php
usort($sv, create_function('$a,$b','return strlen($b) - strlen($a);'));
print_r($sv);
?>
Array
(
  [0] => it is a string thing
  [1] => a big string
  [2] => larger
  [3] => small
)