<?php
function isGreekWord($word) 
{ 
    if (!preg_match_all('#(\p{Greek})#u', $word, $m)) 
        return false; 
    $grTotal = count($m[0]); 
    $enTotal = strlen(iconv('utf-8', 'iso8859-7', $word)) - $grTotal; 
 
    return $grTotal >= $enTotal; 
} 
 
$en = array('a', 'e', 'k'); 
$gr = array('α', 'ε', 'κ'); 
$s = 'kaλημeρa κοσμe hεllo world γαλα'; // string from database 
 
$tokens = explode(' ', $s); 
foreach($tokens as &$token) { 
    if (isGreekWord($token)) 
        $token = str_replace($en, $gr, $token); 
    else 
        $token = str_replace($gr, $en, $token); 
} 
$s = implode(' ', $tokens); 
echo $s;

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