无法将POST内容添加到mysql数据库

Question

<?php

require "config.php";

/*
CREATE TABLE  `addnews` (
 `id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY ,
 `auther` VARCHAR( 255 ) NOT NULL ,
 `title` VARCHAR( 255 ) NOT NULL ,
 `content` LONGTEXT NOT NULL
) ENGINE = MYISAM ;
*/

$a = $_POST['author'];
$t = $_POST['title'];
$c = $_POST['content'];

if(isset($_POST["add"]) and $_POST["add"] == "news"){
    $insert = mysql_query('INSERT INTO addnews 
    (author,title,content)
    VALUES
    ("$a","$t","$c")') or die("error");
    if (isset($insert )){
        echo "<h3>Done</h3>";
    }
};

echo "
<form action='".$_SERVER['PHP_SELF']."' method='post'>
Author : <input type='text' name='author' /><br>
Title : <input type='text' name='title' /><br>
Content : <textarea name='content'></textarea>
<input type='submit' value='Add news' />
<input type='hidden' name='add' value='news' />
</form>
";


mysql_close($connectdb);
?>

i am getting error from this statment i think

if(isset($_POST["add"]) and $_POST["add"] == "news"){
    $insert = mysql_query('INSERT INTO addnews 
    (author,title,content)
    VALUES
    ("$a","$t","$c")') or die("error happend while trying to add information to database");
    if (isset($insert )){
        echo "<h3>Done</h3>";
    }
};

output is : error happend while trying to add information to database

and no problem with config.php file (the file that connect to database) i am using phpmyadmin

Answer 1

1. strings in sql are surrounded by ' (single quote) , not by " (double quote)
2. strings in php will act two ways
   1. those in ' (single quote) will write literally as tyou typed them ($a stays $a - not $a value)
   2. those in " (double quote) will interpret values inside - so $a will be substituted with $a's value
3. when failing DB operation - it is usually useful to see what was wrong - use mysql_error for that