how to update one or more fields ignoring the empty fields into mysql database

$cst = $_POST['custname'];
$a = $_POST['tel'];
$b = $_POST['fax'];
$c = $_POST['email'];
$sql = mysql_query("UPDATE contacts SET TEL = '$a', FAX = '$b', EMAIL = '$c'
                    WHERE Cust_Name = '$cst' ");

$upd = mysql_query("UPDATE custcomm_T SET
Telephone = ".(is_null($a)?'Telephone':"'$a'").",
Fax = ".(is_null($b)?'Fax':"'$b'").",
Mobile = ".(is_null($c)?'Mobile':"'$c'").",
EMail = ".(is_null($d)?'EMail':"'$d'").",
trlicense = ".(is_null($e)?'trlicense':"'$e'").",
trlicexp = ".(is_null($f)?'trlicexp':"'$f'")."
WHERE Cust_Name_VC = '$g' ") or die(mysql_error());

$semaphore = false;
$query = "UPDATE contacts SET ";
$fields = array('tel','fax','email');
foreach ($fields as $field) {
   if (isset($_POST[$field]) and !empty($_POST[$field]) {
     $var = mysql_real_escape_string($_POST[$field]);
     $query .= uppercase($field) . " = '$var'";
     $semaphore = true;
   }
}

if ($semaphore) {
   $query .= " WHERE Cust_Name = '$cst'";
   mysql_query($query);
}

$query = 'update contacts set ';
if ($_POST['tel'] != '') $query .= 'TEL="'.$_POST['tel'].'", ';
if ($_POST['fax'] != '') $query .= 'FAX="'.$_POST['fax'].'", ';
if ($_POST['email'] != '') $query .= 'EMAIL="'.$_POST['email'].'", ';
$query .= "Cust_Name = '$cst' where Cust_Name = '$cst'";

foreach ($_POST as $var=>$value) {
            if(empty($value)) continue; //skip blank fields (may be problematic if you're trying to update a field to be empty)
            $sets[]="$var= '$value";

        }
        $set=implode(', ',$sets);
        $q_save="UPDATE mytable SET $set WHERE blah=$foo";

"UPDATE `custcomm_T`
SET `Telephone` = IF(TRIM('" . mysql_real_escape_string($a) . "') != '', '" . mysql_real_escape_string($a) . "', `Telephone`),
SET `Fax` = IF(TRIM('" . mysql_real_escape_string($b) . "') != '', '" . mysql_real_escape_string($b) . "', `Fax`),
SET `Mobile` = IF(TRIM('" . mysql_real_escape_string($c) . "') != '', '" . mysql_real_escape_string($c) . "', `Mobile`),
SET `EMail` = IF(TRIM('" . mysql_real_escape_string($d) . "') != '', '" . mysql_real_escape_string($d) . "', `EMail`),
SET `trlicense` = IF(TRIM('" . mysql_real_escape_string($e) . "') != '', '" . mysql_real_escape_string($e) . "', `trilicense`),
SET `trlicexp` = IF(TRIM('" . mysql_real_escape_string($f) . "') != '', '" . mysql_real_escape_string($f) . "', `trlicexp`)
WHERE Cust_Name_VC = '" . mysql_real_escape_string($g) . '";

# arrays of input => db field names. If both are the same, no index is required.
$optional = array('tel' => 'telephone', 'fax', 'email');
$required = array('custname' => 'cust_name');

# $input is used rather than $_POST directly, so the code can easily be adapted to
# work with any array.

$input =& $_POST;

/* Basic validation: check that required fields are non-empty. More than is
 necessary for the example problem, but this will work more generally for an
 arbitrary number of required fields. In production code, validation should be
 handled by a separate method/class/module.
 */
foreach ($required as $key => $field) {
    # allows for input name to be different from column name, or not
    if (is_int($key)) {
        $key = $field;
    }
    if (empty($input[$key])) {
        # error: input field is required
        $errors[$key] = "empty";
    }
}
if ($errors) {
    # present errors to user.
    ...
} else {
    # Build the statement and argument array.
    $toSet = array();
    $args = array();
    foreach ($optional as $key => $field) {
        # allows for input name to be different from column name, or not
        if (is_int($key)) {
            $key = $field;
        }
        if (! empty($input[$key])) {
            $toSet[] = "$key = ?";
            $args[] = $input[$key];
        }
    }
    if ($toSet) {
        $updateContactsStmt = "UPDATE contacts SET " . join(', ', $toSet) . " WHERE cust_name = ?";
        $args[] = $input['custname'];
        try {
            $updateContacts = $db->prepare($updateContactsStmt);
            if (! $updateContacts->execute($args)) {
                # update failed
                ...
            }
        } catch (PDOException $exc) {
            # DB error. Don't reveal exact error message to non-admins.
            ...
        }
    } else {
        # error: no fields to update. Inform user.
        ...
    }
}

mysql_query("
    UPDATE contacts
    SET
        TEL = ".(is_null($a)?'TEL':"'$a'").",
        FAX = ".(is_null($b)?'FAX':"'$b'").",
        EMAIL = ".(is_null($c)?'EMAIL':"'$c'")."
    WHERE Cust_Name = '$cst'
");