向数据库中的值添加常量并使用PDO更新

Question

Here is the scenario: I need a mechanism to update a value (adding 1 to the current value) which is stored in a database for a certain user who logs in to the website. This is the current code:

$value = $row["value"];
$add_value = $db->prepare('UPDATE table SET value = $value + 1 WHERE email = :email');
$add_value->execute(array(':email'   => $email));

But what I receive is the following error message: Parse error: syntax error, unexpected T_LNUMBER in ...

What am I doing wrong?

Answer 1

First, you're using single quotes, that means that variables will not be parsed. (The query is literally seeing $value and not the actual value).

Second, you don't need to know the value in advance, the following works fine:

$add_value = $db->prepare('UPDATE `table` SET `value` = `value` + 1 WHERE `email` = :email');
$add_value->execute(array(':email'   => $email));

The database engine knows to update the field appropriately.