<?php

function Clear_Value(&$arg){

    if ($arg == 'NONE') $arg = '';
}

$one    = 'One';
$two    = 'Two';
$three  = 'NONE';
$four   = 'Four';

Clear_Value($one);
Clear_Value($two);
Clear_Value($three);
Clear_Value($four);

echo $one."\n";
echo $two."\n";
echo $three."\n";
echo $four."\n";