<?php
$ipaddress=$_SERVER['REMOTE_ADDR'];
$macaddr=false;

#run the external command, break output into lines

$arp=`arp $ipaddress`;

$lines = explode(" ", $arp);

$macaddr = $lines[3];


?>

<?php
//RETURNS AN IP ADRES 127.0.0.1
$ipaddress=$_SERVER['REMOTE_ADDR'];

$macaddr = false;

#run the external command, break output into lines

$arp='arp ' . $ipaddress;

//You're trying to explode `arp 127.0.0.1` with a space
$lines = explode(" ", $arp);

//$lines contains array(0 => 'arp', 1 => '127.0.0.1')

//$lines[3] does not exist that's where the undefined offset is coming from.
$macaddr = $lines[3];


?>