$username = $_POST['username'];
$password = $_POST['password'];
$result = mysql_query("SELECT * FROM Users WHERE UserName LIKE '%$username%'") or die(mysql_error());

while($row = mysql_fetch_array($result))
{
    echo $row['FirstName'];
}

有时将查询抑制为@mysql_query(您的查询);

因为$username是一个PHP变量，我们需要将它作为字符串传递给mysqli，所以在查询中，你以单引号开始，我们将使用双引号，单引号和句号来连接("'.$username.'")，如果你以双引号开始，你将反转引号('".$username."')。

$username = $_POST['username'];
$password = $_POST['password'];
$result = mysql_query('SELECT * FROM Users WHERE UserName LIKE "'.$username.'"');

while($row = mysql_fetch_array($result))
     {
      echo $row['FirstName'];
     }

$username = $_POST['username'];
$password = $_POST['password'];
$result = mysql_query("SELECT * FROM Users WHERE UserName LIKE '".$username."' ");

while($row = mysql_fetch_array($result))
     {
      echo $row['FirstName'];
     }

但是Mysql的使用已经贬值了很多，改用PDO。它很简单，但非常安全

您的代码应该是这样的

$username = $_POST['username'];
$password = $_POST['password'];
$query = "SELECT * FROM Users WHERE UserName LIKE '$username'";
echo $query;
$result = mysql_query($query);

if($result === FALSE) {
    die(mysql_error("error message for the user")); 
}

while($row = mysql_fetch_array($result))
{
    echo $row['FirstName'];
}

完成此操作后，您将在屏幕上显示查询。在服务器上尝试这个查询，看看它是否产生了所需的结果。大多数情况下，错误出现在查询中。其余的代码是正确的。

可能有两个原因:

在调用mysql_query函数之前，是否打开了到数据库的连接?我在你的代码里没看到。在进行查询之前使用mysql_connect。看到php.net/manual/en/function.mysql-connect.php 变量$username在单引号字符串中使用，因此它的值不会在查询中计算。查询肯定会失败。

第三，查询结构容易被SQL注入。您可以使用准备好的语句来避免这种安全威胁。

当查询中出现错误导致查询失败时，将显示此错误消息。它会在使用时显现出来:

mysql_fetch_array / mysqli_fetch_array () 作用是()/ mysqli_fetch_assoc () mysql_num_rows () / mysqli_num_rows ()

注意:如果查询不影响任何行，则不会出现此错误。只有语法无效的查询才会产生此错误。

故障排除步骤

Make sure you have your development server configured to display all errors. You can do this by placing this at the top of your files or in your config file: error_reporting(-1);. If you have any syntax errors this will point them out to you. Use mysql_error(). mysql_error() will report any errors MySQL encountered while performing your query. Sample usage: mysql_connect($host, $username, $password) or die("cannot connect"); mysql_select_db($db_name) or die("cannot select DB"); $sql = "SELECT * FROM table_name"; $result = mysql_query($sql); if (false === $result) { echo mysql_error(); } Run your query from the MySQL command line or a tool like phpMyAdmin. If you have a syntax error in your query this will tell you what it is. Make sure your quotes are correct. A missing quote around the query or a value can cause a query to fail. Make sure you are escaping your values. Quotes in your query can cause a query to fail (and also leave you open to SQL injections). Use mysql_real_escape_string() to escape your input. Make sure you are not mixing mysqli_* and mysql_* functions. They are not the same thing and cannot be used together. (If you're going to choose one or the other stick with mysqli_*. See below for why.)

其他技巧

Mysql_ *函数不应该用于新代码。它们不再被维护，社区已经开始了弃用过程。相反，你应该学习准备语句并使用PDO或MySQLi。如果你不能决定，这篇文章将帮助你选择。如果你想学习，这里有一个很好的PDO教程。

如果检查时没有出现任何MySQL错误，请确保正确创建了数据库表。这发生在我身上。寻找任何不需要的逗号或引号。