我正在尝试编写一个代码，其中所有SQL语句都存储在一个单独的PHP文件中，以便以后可以将它们作为函数来调用。

我尝试了不同的方法，但似乎无法使其正常工作。

这是我的代码（我正在尝试在此处创建登录页面）

login.php

<?php
     include('sql.php');
     $sql = new sql();
?>

<form class = "form-inline" method = "post" style="color: #FFF; position: absolute; margin-top:100px; margin-left:500px;">
            <table width="100%" border="0">
  <tr>
    <td>Username</td>
    <td><input type="text" id = "username" name = "username" class="large" placeholder=""></td>
  </tr>
  <tr>
    <td>Password</td>
    <td><input type="password" id = "password" name = "password" class="large" placeholder=""></td>
  </tr>
</table>
<br />
<center><button type="submit" id = "submit" name = "submit" class="btn btn-primary">Log in</button></center>
</form>
<?php
//echo $sql->admin();
?>


<?php
    if(isset($_POST['submit'])) {
        // username and password sent from form
        $username=$_POST['username'];
        $password=$_POST['password'];

        // To protect MySQL injection (more detail about MySQL injection)
        $username = stripslashes($username);
        $password = stripslashes($password);
        $username = mysql_real_escape_string($username);
        $password = mysql_real_escape_string($password);

        //THIS IS THE PART WHERE I CALL THE SQL STATEMENT FROM A SEPARATE PHP FILE
                            echo $sql->admin($username,$password);

        // Mysql_num_row is counting table row
        $count=mysql_num_rows($sql);

        // If result matched $username and $password, table row must be 1 row               
        if($count==1)
        {
            //unset($_SESSION);
            $row = mysql_fetch_assoc($sql);
            $_SESSION[logged] = $row[logged];
            // Register $username, $password and redirect to file "index.php"
            $_SESSION['username'] = $username;
            $_SESSION['password'] = $password;
            $_SESSION['submitted'] = $row[submitted];
            $_SESSION['date_submitted'] = $row[date_submitted];
            session_register($_SESSION['username']);
            session_register($_SESSION['password']);
            ?>
                                     <script>window.location="index.php";</script>
            <?php
        }
        else 
        { ?>
            <center><span style="color:white;">Wrong Username or Password</span></center>
        <?php

        }
    }
?>

sql.php

<?php
  include('connect.php');
  class sql{
    function sql()
    {

    }

    function admin($username,$password)
    {
        echo $sql=mysql_query("SELECT * FROM admin WHERE admin_username= '$username' and admin_password= '$password' ");
    } 
}
?>

此代码出现的错误是

Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in login.php on line 40

我的40号线是

$count=mysql_num_rows($sql);

我终于明白了 如您所说，我在sql.php中添加了一行

    return $result;

我的代码无法正常工作的另一个原因是，我没有放置自己的代码

 $sql->admin($username,$password);

里面的一个变量。现在看起来像这样

$a = $result->admin($username,$password);

现在正在工作。