You need quotation marks:
<?php
if(isset($_GET['id']) && is_numeric($_GET['id']))
{
$job_id = (int) $_GET['id'];
}
$sql = "UPDATE edworld_jobs SET (
contactname = \"$_POST['contactname']\", //THIS IS LINE 15
emailaddress = \"$_POST['emailaddress']\",
telephone = \"$_POST['telephone']\",
jobref = \"$_POST['jobref']\",
jobtitle = \"$_POST['jobtitle']\",
startdate = \"$_POST['startdate']\",
deadline = \"$_POST['deadline']\",
category = \"$POST['category']\",
jobdescription = \"$_POST['jobdescription']\",
location = \"$_POST['location']\",
hours = \"$_POST['hours']\",
contract = \"$_POST['contract']\",
salary = \"$_POST['salary']\"
WHERE JobID = $job_id";
?>