<?php

$groups="group1,, group3";
$functions="member,, admin";

$groups_array = array_map('trim',explode(',', $groups));
$functions_array = array_map('trim',explode(',', $functions));

$final = array();
for($i = 0; $i <= count($groups_array); $i++)
{
  $final[$groups_array[$i]] = $functions_array[$i];
}

$merged = array_combine($groups_array, $functions_array);

echo "with a for:\n";
print_r($final);
echo "\nusing array_combine:\n";
print_r($merged);

with a for:
Array
(
    [group1] => member
    [] => 
    [group3] => admin
)

using array_combine:
Array
(
    [group1] => member
    [] => 
    [group3] => admin
)