MySQL的PHP​​变数（SET'$ field'='$ value'）

Question

I'm trying to use PHP variables in a MySQL query like this:

$field = "name_of_field";
$value = "new_value";
$sql = "UPDATE table SET '$field' = '$value'";

It doesn't work. When I take out the $field var and just use the name of a field it works.

Answer 1

Remove the quotes around the column name.

$field = "name_of_field";
$value = "new_value";
$sql = "UPDATE table SET $field = '$value'";

or use backticks

$field = "name_of_field";
$value = "new_value";
$sql = "UPDATE table SET `$field` = '$value'";

Using backticks around column names ( and/or tables ) ensures that you've not used a (mysql) reserved word as well as using spaces in between words. This practice is discouraged.

For example: (here are a few examples of reserved words) - try not using those for tables and/or column names, should this happen to occur anytime in the future.

• for
• to
• as
• desc
• is
• etc .

For a list of reserved words, consult the following page on mysql.com :

• http://dev.mysql.com/doc/refman/5.5/en/reserved-words.html

Example of spaces between words for tables and columns: (while using backticks)

SELECT `column one` FROM `my table`

which would be valid.

Yet, by using the following

SELECT column one FROM my table

would produce an error.

It's best to use underscores and not hyphens ( should this also be the case for future development ). Using hyphens is discouraged to use as a word seperator, as SQL will misinterpret the hyphen as a minus sign, in thinking you want to do some form of mathematical problem.

---

When in development: (use error reporting)

error_reporting(E_ALL);
ini_set('display_errors', 1);

and the following if using mysqli_ functions:

mysqli_report(MYSQLI_REPORT_ERROR | MYSQLI_REPORT_STRICT);

MySQL (error reporting links)

• http://www.php.net/manual/en/function.mysql-error.php
• http://www.php.net/manual/en/mysqli.error.php
• http://www.php.net/mysqli_error

PDO

• http://www.php.net/manual/en/pdo.error-handling.php
• http://www.php.net/manual/en/pdo.errorinfo.php

Answer 2

您不会在字段名称周围使用单引号，而使用反引号`

Answer 3

Two things wrong.

1. Firstly, you need to wrap your value for $sql in quotes.
2. You need to remove the single quotes around $field
3. You can wrap $field in curly braces (optional)

Result:

$field = "name_of_field";
$value = "new_value";
$sql   = "UPDATE table SET `{$field}` = '$value'";