Another generic MySQL error w/ PHP PDO - php

Question

I'm getting MySQL error 42000:1064 that suggests a general syntax error with the following SQL:
UPDATE `events` SET ?=?, ?=?, ?=now() WHERE `event_id`=?;
PHP code to convert to a readable statement & also execute:
<?php
$ar = array_fill(0,count($args),'/\?/');
echo preg_replace($ar,$args,$sql,1);
$this->execute($sql, $args);
?>
This evaluates to:
UPDATE `events` SET event_name=test, form_id=webform, last_updated=now() WHERE `event_id`=124;
Which when pasted into the MySQL workbench completes successfully.
[mysqlErrorMsg] => SQLSTATE[42000]: Syntax error or access violation: 1064 You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''event_name'='test', 'form_id'='webform', 'last_updated'=now() WHERE `event_id`=' at line 1
It should be noted that my user has full access to the table in question.

Answer 1

You can't use placeholders on column names. Only on values.
Your query does NOT evaluate to (as it should)
UPDATE `events` SET event_name=test, form_id=webform, last_updated=now()
WHERE `event_id`=124;
but is being evaluated as this instead:
UPDATE `events` SET 'event_name'='test', 'form_id'='webform', 'last_updated'=now()
WHERE `event_id`=124;
See the quotes? These are strings, not column names.
So hard code the column names and only use placeholders for values
UPDATE `events` SET event_name=?, form_id=?, last_updated=now() WHERE `event_id`=?;