<?php
$array = array(
'Element1', 
'Element2', 
'Element3', 
'Element1', 
'Element1', 
'Element4', 
'Element4', 
'Element2', 
'Element2', 
'Element2', 
'Element2', 
'Element4', 
'Element5', 
'Element5' );
$numbers = array_count_values($array);
asort($numbers);
$result = array_keys($numbers);
print_r($result);
?>

Array
(
    [0] => Element3
    [1] => Element5
    [2] => Element4
    [3] => Element1
    [4] => Element2
)