<?php
$arr = array('one' => 'Value1', 'two' => 'Value2', 'three' => 'Value3');

function moveToHead($array,$key) {
 $return = $array;
 $add_val = array($key => $return[$key]);
 unset($return[$key]);
 return $add_val + $return;
}

print_r(moveToHead($arr,'two'));
 
?>

Array
(
    [two] => Value2
    [one] => Value1
    [three] => Value3
)