<?php
$macArr = file('./mac.txt' );  
foreach($macArr as $mac ) { 
    $mac = trim($mac ) ; 
    $mArr = preg_split("//u", $mac, 0, PREG_SPLIT_NO_EMPTY );
    $tmpArr = array_chunk($mArr, 2 ) ; 
    foreach($tmpArr as & $i ) { 
        $i = implode('', $i ) ; 
    }   
    $tmpArr = array_filter($tmpArr) ;
    $tmp = implode(':', $tmpArr ) ; 
    echo $tmp."\n"; 
}