<?php

$arr = array(3350, 
    'Williamson New England Electric', 
    'bob@jojozzz.go', 
    'Pumps & Motors,Electronic Systems,Electrical Supply,Electrical '
);

array_splice($arr, -1, 1, explode(',', end($arr)));


print_r($arr);

Array
(
    [0] => 3350
    [1] => Williamson New England Electric
    [2] => bob@jojozzz.go
    [3] => Pumps & Motors
    [4] => Electronic Systems
    [5] => Electrical Supply
    [6] => Electrical 
)